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L.  i  u  i\  n 


THE 


GAS     ENGINE 


PRINTED    BY 

8POTTISWOODE    AND    CO.,    NEW-STREET   SQUARE 
LONDON 


THE 


GAS     ENGINE 


BY 


DUGALD     CLERK 


FOURTH    EDITION 


NEW   YORK 
JOHN     WILEY     &     SONS 

53    EAST    TENTH    STREET 
1893 


PREFACE. 


IN  this  work  the  author  has  endeavoured  to  systematise  the  know- 
ledge in  existence  upon  the  subject,  and  to  explain  the  science 
and  practice  of  the  Gas  Engine  in  a  way  which  he  hopes  may  be 
useful  to  the  engineer. 

The  historical  sketch  with  which  the  book  opens  proves  that, 
like  other  great  subjects,  the  .gas  engine  has  long  occupied  men's 
minds. 

The  first  six  chapters  treat  of  theory,  including  the  distin- 
guishing features  of  the  gas  engine  method,  classification,  thermo- 
dynamics of  the  various  types,  and  the  chemical  and  physical 
phenomena  of  combustion  and  explosion. 

In  the  seventh  chapter,  standard  engines  illustrative  of  the 
different  types  are  described,  and  tests  from  each  engine  for  power 
and  consumption  of  gas  are  given.  The  diagrams  and  efficiencies 
are  shortly  discussed,  compared  with  theory,  and  the  various  sources 
of  loss  pointed  out. 

The  eighth  chapter  deals  with  typical  igniting  arrangements, 
and  the  ninth  with  governing  gear  and  other  mechanical  details. 


vi  Preface 

The  tenth  chapter  briefly  describes  and  discusses  various 
theories  which  have  been  propounded  concerning  the  action  of 
the  gases  in  the  cylinder  of  the  gas  engine  and  in  gaseous  explo- 
sions. 

In  the  last  chapter  the  great  sources  of  loss  of  heat  still 
existing  in  the  best  gas  engines  are  discussed,  with  the  object  of 
pointing  out  the  way  still  open  for  further  advance. 

Many  of  the  tests  and  most  of  the  theoretical  and  practical 
discussion,  result  from  the  author's  personal  experience  with  the 
gas  engine. 

In  the  chapter  on  thermodynamics  the  author  is  much  indebted 
to  the  work  of  the  late  Prof.  RANKINE,  and  he  has  adopted,  in 
treating  of  efficiency,  some  of  the  elegant  formulae  of  Dr.  AIME 
\YITZ,  of  Lille,  to  whom  as  well  as  to  Prof.  SCHOTTLER  and  Prof. 
THURSTON  he  has  much  pleasure  in  expressing  his  indebtedness. 

D.  C 

BIRMINGHAM  :  July  1886. 


CONTENTS. 


CHAPTER  PAGE 

HISTORICAL  SKETCH  OF  THE  GAS  ENGINE,  1690  TO  1885  .      i 

I.     THE  GAS  ENGINE  METHOD 23 

II.     GAS  ENGINES  CLASSIFIED       .......     29 

III.  THERMODYNAMICS  OF  THE  GAS  ENGINE 36 

IV.  THE  CAUSES  OF  Loss  IN  GAS  ENGINES        ...        .72 
V.     COMBUSTION  AND  EXPLOSION 79 

VI.     EXPLOSION  IN  A  CLOSED  VESSEL 95 

VII.     THE  GAS  ENGINES  OF  THE  DIFFERENT  TYPES  IN  PRACTICE  116 
VIII.     IGNITING  ARRANGEMENTS 202 

IX.      ON   SOME   OTHER   MECHANICAL   DETAILS  ....    226 

X.    THEORIES  OF  THE  ACTION  OF  THE  GASES  IN  THE  MODERN- 
GAS  ENGINE 243 

XI.    THE  FUTURE  OF  THE  GAS  ENGINE 260 

APPENDIX 271 

INDEX 275 


THE 

^        GAS    ENGINE. 

HISTORICAL   SKETCH  OF  THE   GAS  ENGINE. 

THE  origin  of  the  gas  engine  is  but  imperfectly  known  ;  by  some 
it  is  dated  as  far  back  as  1680,  when  Huyghens  proposed  to  use 
gunpowder  for  obtaining  motive  power.  Papin,  in  1690,  continued 
Huyghens'  experiments,  but  without  success.  The  method  used 
was  a  fairly  practicable  one.  The  explosion  was  used  indirectly  ; 
a  small  quantity  of  gunpowder  exploded  in  a  large  cylindrical  vessel 
filled  with  air,  expelled  the  air  through  check  valves,  thus  leaving, 
after  cooling,  a  partial  vacuum.  The  pressure  of  the  atmosphere 
then  drove  a  piston  down  to  the  bottom  of  the  vessel,  lifting  a 
weight  or  doing  other  work. 

In  a  paper,  published  at  Leipsic  in  1688,  Papin  stated  that, 
'  until  now  all  experiments  have  been  unsuccessful ;  and  after  the 
combustion  of  the  exploded  powder,  there  always  remains  in  the 
cylinder  about  one-fifth  of  its  volume  of  air.' 

The  Abbe  Hautefeuille  made  similar  proposals,  but  does  not 
seem  to  have  made  actual  experiments.  These  early  engines 
cannot  be  classed  as  gas  engines.  The  explosion  of  gunpowder  is 
so  different  in  its  nature  from  that  of  a  gaseous  mixture  that  com- 
parison is  untenable.  The  first  real  gas  engine  described  in  this 
country  is  in  Robert  Street's  patent,  No.  1983,  1794.  It  contains  a 
motor  cylinder  in  which  works  a  piston  connected  to  a  lever, 
from  which  lever  a  pump  is  driven.  The  bottom  of  the  motor 
cylinder  is  heated  by  a  fire  ;  a  few  drops  of  spirits  of  turpentine 


2  Tfie  Gas  Engine 

being  introduced  and  evaporated  by  the  heat,  the  motor  piston 
is  drawn  up,  and  air  entering  mixes  with  the  inflammable  vapour, 
the  application  of  a  flame  to  a  touch-hole  causing  explosion  :  and 
the  piston  being  driven  up  forces  the  pump  piston  down,  so  per- 
forming work  in  raising  water.  The  details,  as  described,  are 
crude,  but  the  main  idea  is  correct  and  was  not  improved  upon  in 
practice  till  very  lately. 

Samuel  Brown's  inventions  come  next.     His  patents  are  dated 
1823  and  1826,  Nos.  4874  and  5350.     The  principle  used  is  in- 


A,  Cover  raised,  vessel  filling  with  flame. 


B  and  C,  Covers  down,  vessels  vacuous. 


FlG.  I. — Brosvn's  Gas-vacuum  Engine,  1826. 

genious,  and  easily  carried  out  in  practice,  but  it  is  not  economical, 
and  it  gives  a  very  cumbrous  machine  for  the  amount  of  power 
produced.  A  partial  vacuum  is  produced  by  filling  a  vessel  with 
flame,  and  expelling  the  air  it  contains,  a  jet  of  water  is  thrown 
in  and  condenses  the  flame,  giving  vacuum.  The  atmospheric 
pressure  thus  made  available  for  power  is  utilised  in  any  engine  of 
ordinary  construction. 

Hrown's  apparatus  consists  essentially  of  a  large  upright  cylin- 


Historical  Sketch  of  the  Gas  Engine  3 

drical  vessel  fitted  on  the  top  with  a  movable  valve  cover,  of  the 
whole  diameter  of  the  cylinder.     The  cover  is  raised  and  lowered 
from  and  to  its  seat  by  a  lever  and  suitable  gear  at  proper  times. 
The   gas  supply  pipe  enters   the   cylinder   at  the    bottom  ;   the 
cylinder   being   filled   with   air,  and   the   valve   raised,    the   gas 
cock   is  opened   and   the   issuing  gas  lighted  by  a  small   flame 
as  it  enters  the  cylinder.      The  flame  produced  fills  the  whole 
vessel,    expelling    the    air   it    contains  ;    the    valve    being   now 
lowered  and  the  gas  supply  shut  off,  the  water-jet  is  thrown  in 
and  causes  condensation.     To  keep  up  a  constant  supply  of  power, 
several  of  these  cylinders  are  required,  so  that  one  at  least  may  be 
always  vacuous  while  the  others  are  in  the  process  of  obtaining 
the  vacuum.      In  the  specification  three  are  shown  and   three 
engines.     The  engines  are  all  connected  to  the  same  crank-shaft. 
Notwithstanding   this   provision,   the    motion    must   have    been 
irregular.     The  idea  was  evidently  suggested  by  the  condensing 
steam  engine  ;  instead  of  using  steam  to  obtain  a  vacuum  flame  is 
employed.     Brown's  engine,  although  uninteresting  theoretically, 
is  important  as  being  the  first  gas  engine  undoubtedly  at  work. 
According  to  the  '  Mechanics'  Magazine,'  published  in  London,  a 
boat  was  fitted  with  one  including   a  complete  gas  generating 
plant,  and  was  run  upon  the  Thames  not  for  public  use  but  only 
as  an  experiment.      Another  engine  was  made  in   combination 
with  a  road  carriage  ;  it  also  ran  in  London.     If  these  statements 
are  to  be  relied  upon,  then  Samuel  Brown  was  a  really  great  man 
and  should  be  considered  as  the  Newcomen  of  the  gas  engine  ;  in 
some  points  he  achieved  a  measure  of  success  not  yet  equalled  by 
his  successors. 

W.  L.  \Vright,  1833,  No.  6525.— In  this  specification  the 
drawings  are  very  complete  and  the  details  are  carefully  worked 
out.  The  explosion  of  a  mixture  of  inflammable  gas  and  air  acts 
directly  upon  the  piston,  which  acts  through  a  connecting  rod  upon 
a  crank-shaft.  The  engine  is  double-acting,  the  piston  receiving 
two  impulses  for  every  revolution  of  the  crank-shaft.  In  appear- 
ance it  resembles  a  high  pressure  steam  engine  of  the  kind  known 
as  the  table  pattern.  The  gas  and  air  are  supplied  to  the  motor 
cylinder  from  separate  pumps  through  two  reservoirs,  at  a  pressure 
a  few  pounds  above  atmosphere,  the  gases  (gas  and  air)  enter 


4  The  Gas  Engine 

spherical  spaces  at  the  ends  of  the  motor  cylinder,  partly  displacing 
the  previous  contents,  and  are  ignited  while  the  piston  is  crossing 
the  dead  centre.  The  explosion  pushes  the  piston  up  or  down 
through  its  whole  stroke  ;  at  the  end  of  the  stroke  the  exhaust 
valve  opens  and  the  products  of  combustion  are  discharged  during 


FlG.  2.— Wright's  Gas-exploding  Engine,  1833. 

the  return,  excepting  the  portion  remaining  in  the  spaces  not 
entered  by  the  piston.  The  ignition  is  managed  by  an  external 
flame  and  touch-hole.  The  author  has  been  unable  to  find 
whether  the  engine  was  ever  made,  but  the  knowledge  of  the 
detail  essential  to  a  working  gas  engine  shown  by  the  drawings 
indicates  that  it  or  some  similar  machine  had  been  worked  by  the 


Historical  Sketch  of  the  Gas  Engine  5 

inventor.  Both  cylinder  and  piston  are  water-jacketed,  as  would 
have  been  necessary  in  a  double-acting  gas  engine  to  preserve  the 
working  parts  from  damage  from  the  intense  heat  of  the  explosion. 
This  is  the  earliest  drawing  in  which  this  detail  is  properly  shown. 
William  Barnett,  1838,  No.  7615. — Barnett's  inventions  as 
described  in  his  specification  are  so  important  that  they  require 
more  complete  description  than  has  been  here  accorded  to  earlier 
inventors. 

Barnett  is  the  inventor  of  a  very  good  form  of  igniting  arrange- 
ment. The  flame  method  most  widely  used  at  the  present  time 
was  originated  by  him. 

Barnett  is  also  the  inventor  of  the  compression  system  now  so 
largely  used  in  gas  engines.  The  Frenchman,  Lebon,  it  is  true, 
described  an  engine  using  compression,  in  the  year  1801,  but  his 
cycle  is  not  in  any  way  similar  to  that  proposed  by  Barnett,  or 
used  in  the  modern  gas  engine.  Barnett  describes  three  engines. 
The  first  is  single-acting,  the  second  and  third  are  double-acting  ; 
all  compress  the  explosive  mixture  before  igniting  it.  In  the  first 
and  second  engines  the  inflammable  gas  and  air  is  compressed  by 
pumps  into  receivers  separate  from  the  motor  cylinder,  but  com- 
municating with  it  by  a  short  port  which  is  controlled  by  a  piston 
valve.  The  piston  valve  also  serves  to  open  communication 
between  the  cylinder  and  the  air  when  the  motor  piston  dis- 
charges the  exhaust  gases. 

In  the  third  engine  the  explosive  mixture  is  introduced  into 
the  motor  cylinder  by  pumps,  displacing  as  it  enters  the  exhaust 
gases  resulting  from  the  previous  explosion  ;  the  motor  piston  by 
its  ascent  or  descent  compresses  the  mixture.  Part  of  the  com- 
pression is  accomplished  by  the  charging  pumps,  but  it  is  always 
completed  in  the  motor  cylinder  itself. 

In  all  three  engines  the  ignition  takes  place  when  the  crank  is 
crossing  the  dead  centre,  so  that  the  piston  gets  the  impulse  during 
the  whole  forward  stroke. 

Fig.  3  is  a  sectional  elevation  of  the  first  engine,  showing  the 
principal  working  parts,  but  omitting  all  detail  not  required  for 
explaining  the  action. 

There  are  three  cylinders  containing  pistons  ;  A  is  the  motor 
piston,  B  is  the  air  pump  piston.  The  gas  pump  piston  cannot  be 


6  The  Gas  Engine 

seen  in  the  section,  but  works  in  the  same  crosshead  as  E.  The 
motor  piston  is  suitably  connected  to  the  crank  shaft,  and  the 
other  two  are  also  connected  by  levers  in  such  manner  that  all 
three  move  simultaneously  up  or  down.  The  pump  pistons, 
moving  up,  take  respectively  air  and  inflammable  gas  into  their 


FIG.  3.— Barnett  Gas  Engine. 

cylinders  ;  upon  the  down  stroke  the  gases  are  forced  through  an 
automatic  lift  valve  into  the  receiver  D,  and  there  mix.  When  the 
down  stroke  is  complete  and  the  receiver  is  fully  charged  with  the 
explosive  mixture,  the  pressure  has  risen  to  about  25  Ibs.  per  square 
inch  above  atmosphere.  At  the  same  time  as  the  pumps  are  com- 
pressing, the  motor  piston  is  moving  down  and  discharging  the 


Historical  Sketch  of  the  Gas  Engine  7 

exhaust  gases  from  the  power  cylinder;  it  reaches  the  bottom  of 
its  stroke  just  when  compression  is  complete.  The  piston  valve 
E  then  opens  communication  between  the  receiver  and  the  motor, 
at  the  same  time  closing  to  atmosphere.  The  motor  cylinder 
being  in  free  communication  with  the  receiver,  the  explosion  of 
the  mixture  is  accomplished  by  the  igniting  cock  or  valve  F  ;  the 
pressure  resulting  actuates  the  motor  piston  during  its  whole  up- 


FIG.  4'— Barnett's  Igniting  Cock. 

ward  stroke,  the  hot  gases  flowing  through  the  port  G  precisely  as 
steam  would  do.  The  volume  of  the  receiver  being  constant,  the 
pressure  in  the  motor  cylinder  slowly  falls  by  expansion,  due  to 
the  movement  of  the  piston,  upon  which  work  is  performed,  and 
by  cooling,  the  pressure  still  existing  in  the  cylinder  when  the 
stroke  is  complete  depending  on  the  ratio  between  the  volume 
swept  by  the  motor  piston  and  the  volume  of  the  receiver. 


8  The  Gas  Engine 

The  down  stroke  again  expels  the  products  of  combustion,  the 
valve  opening  to  atmosphere,  while  the  compression  again  takes 
place.  This  cycle  gives  a  single-acting  engine.  It  is  obvious  that  as 
the  piston  A  does  not  enter  the  receiver  it  cannot  displace  the 
exhaust  gases  there.  If  means  are  not  taken  to  expel  these  gases 
they  must  mix  with  the  fresh  explosive  charge  pumped  in. 

It  is  very  desirable  that  these  gases  should  be  as  completely 
as  possible  discharged.  An  exhausting  pump  is  described  for 
doing  this,  but  in  small  engines  it  adds  an  additional  complication  ; 
and  so  Barnett  states  that  in  some  cases  it  may  be  omitted.  The 
exhaust  gases  do  not  so  injuriously  affect  the  action  of  small  gas 
engines. 

The  igniting  valve  is  very  ingenious.  It  is  shown  at  Fig  4,  on  a 
larger  scale.  A  hollow  conical  plug  A  is  accurately  ground  into 
the  shell  B,  and  is  kept  in  position  by  the  gland  c  ;  the  shell  has 
two  long  slits  D  and  E  ;  the  plug  has  one  port  so  cut  that  as  the 
plug  moves  it  shuts  to  the  slit  D  before  opening  to  E.  In  the 
bottom  of  the  shell  there  is  screwed  a  cover  carrying  a  gas  burner 
F,  which  may  be  lit  while  the  port  in  the  plug  is  open  to  the  air 
through  D.  The  external  constant  flame  H  lights  it.  So  long 
as  the  plug  remains  in  this  position  the  internal  flame  continues 
to  burn  quietly.  If  the  plu  *  be  now  turned  to  shut  to  the  outer 
air,  it  opens  to  the  slit  E,  and  as  that  contains  explosive  mixture 
it  at  once  ignites.  The  explosion  extinguishes  the  internal  flame, 
but  it  is  again  lighted  at  the  proper  time  when  the  plug  is  moved 
round.  The  valve  acts  well  and  is  almost  identical  in  principle 
with  the  flame-igniting  arrangements  of  Hugon,  Otto  and  Langen 
and  Otto. 

Barnett's  second  engine  is  identical  with  his  first  except  that 
it  is  double-acting,  and  therefore  requires  a  greater  number  of 
parts. 

Barnett's  third  engine  is  worthy  of  careful  description.  Fig.  5 
is  a  vertical  section  of  the  principal  parts.  It  is  double-acting. 
It  has  three  cylinders,  motor,  air-pump  and  gas-pump  ;  the  air 
and  gas  pumps  are  single  acting,  the  motor  piston  is  double- 
acting.  The  pumps  are  driven  from  a  separate  shaft,  which  is 
actuated  from  the  main  crank  shaft  by  toothed  wheels  ;  the  wheel 
upon  the  pump  shaft  is  half  the  diameter  of  that  on  the  motor 


Historical  Sketch  of  the  Gas  Engine  9 

shaft,  so  that  it  makes  two  revolutions  for  one  of  the  other.  The 
pumps  therefore  make  one  up-and-down  stroke  for  each  up  or 
down  stroke  of  the  motor  piston  ;  the  angles  of  the  cranks  are  so 
set  that  they  (pumps)  discharge  their  contents  into  one  or  other  side 
of  the  motor  cylinder  at  every  stroke  ;  the  exhaust  gases  are  panly 


FIG.  5. — Barnett  Engine. 

displaced  by  the  fresh  explosive  mixture,  and  the  motor  piston' com- 
pletes the  compression  in  the  motor  cylinder  itself.  When  full  up 
or  down  the  igniting  cock  acts,  and  the  explosion  drives  the  piston 
to  the  middle  of  its  stroke  ;  it  here  runs  over  a  port  in  the  middle 
of  the  cylinder,  and  the  pressure  at  once  falls  to  atmosphere. 


10  The  Gas  Engine 

A  is  the  motor  piston  ;  B  is  the  air-pump  piston  ;  c  is  the  gas- 
pump  piston,  which  is  behind  the  air-pump,  and  therefore  not 
seen  in  the  section  ;  D  is  the  main  crank  shaft ;  E  the  pump  shaft 
driven  from  the  main  shaft  by  the  wheels  F  and  G.  The  engine 
is  exceedingly  interesting  as  the  first  in  which  the  compression  is 
accomplished  in  the  motor  cylinder,  but  it  is  not  so  good  a  machine 
as  the  first  because  of  the  difficulty  of  obtaining  a  sufficient  amount 
of  expansion. 

From  1838  to  1854  inclusive  eleven  British  patents  were  applied 
for  ;  some  were  not  completed  but  only  reached  the  provisional 
stage.  Of  these  patents  by  far  the  most  important  is  Barnett's  ;  the 
others  are  interesting  as  showing  the  gradual  increase  of  attention 
the  subject  attracted.  The  other  names  are  Ador,  1838  ;  Johnson, 
1841  ;  Robinson,  1843  ;  Reynolds,  1844  ;  Brown,  1846  :  Roger, 
1853,  also  Bolton  and  Webb,  making  three  patents  for  the  year  ; 
for  1854  two  patents,  Edington  and  Barsanti  and  Matteucci.  None 
of  the  proposals  in  these  patents  are  really  valuable  or  novel, 
being  anticipated  by  either  Street,  Wright,  or  Samuel  Brown. 
Robinson's  is  the  best,  being  similar  to  Lenoir's  in  some  of  its 
details,  and  showing  distinctly  a  better  understanding  of  gas 
engine  detail. 

A.  V.  Nwvton,  1855,  No.  562. — This  specification  is  interesting, 
and  describes  for  the  first  time  a  form  of  igniting  arrangement 
only  now  coming  into  use  ;  it  seems  to  be  identical  with  the 
invention  of  the  American  Drake,  although  not  described  as  a 
communication  from  him.  It  is  a  double-acting  engine,  and  takes 
into  the  cylinder  a  charge  of  gas  and  air  mixed,  during  a  portion 
of  the  stroke,  at  atmospheric  pressure.  The  igniting  arrangement 
is  a  thimble-shaped  piece  of  hard  cast-iron  which  projects  into  a 
recess  formed  in  the  side  of  the  cylinder  ;  it  is  hollow,  and  is  kept 
at  all  times  red-hot  by  a  blow-pipe  flame  projected  into  it  by  a  small 
pump.  When  the  piston  uncovers  the  recess  the  explosive  gases 
coming  in  contact  with  it  ignite,  and  the  pressure  produced  drives 
it  forward. 

This  is  the  first  instance  of  ignition  by  contact  with  red-hot 
metal ;  the  proposal  has  often  been  made  since  then  in  varying 
forms. 

Barsanti  and  Matteucci,  1857,  No.  1655. — This  is  the  first  free 


Historical  Sketch  of  the  Gas  Engine  1 1 

piston  engine  ever  proposed  ;  instead  of  allowing  the  explosion  to  act 
directly  upon  the  motive  power  shaft  through  a  connecting  rod, 
at  the  moment  of  explosion  the  piston  is  perfectly  free.  The 
cylinder  is  very  long,  and  is  placed  vertically.  When  the  explosion 
occurs,  it  expends  its  power  in  giving  the  piston  velocity  ;  the 
expansion  therefore  takes  place  with  considerable  rapidity,  and 
the  piston,  gaining  speed  until  the  pressure  upon  it  falls  to  atmo- 
sphere, moves  on,  till  the  energy  of  motion  is  absorbed  doing  work 
on  the  external  air,  lifting  the  piston  and  in  friction.  When  the 
energy  is  all  absorbed  in  this  manner  it  stops  ;  it  has  reached  the 
top  of  its  stroke.  A  partial  vacuum  has  been  formed  in  the  cylinder, 
and  the  weight  has  been  raised  through  the  stroke.  It  now 
returns  under  the  pressure  of  the  atmosphere  and  its  own  weight ; 
in  returning,  a  rack  attached  to  the  piston  engages  the  motive  shaft 
and  drives  it.  The  cooling  of  the  gases  as  the  piston  descends 
continues  and  helps  to  keep  up  the  vacuum. 

The  method  although  indirect  is  economical.  Three  advantages 
are  gained  by  it — rapid  expansion,  considerable  expansion  (an  ex- 
pansion of  six  times  is  common  in  these  engines),  and  also  some 
of  the  advantages  of  a  condenser. 

Fig.  6  shows  a  vertical  section  of  their  best  modification.  The 
motor  piston  A  working  in  the  tall  vertical  cylinder  B  is  attached  to 
the  rack  c,  which  works  into  the  toothed  wheel  D.  The  motor 
shaft  E  revolves  in  the  direction  of  the  arrow,  and  it  is  provided 
with  a  ratchet ;  a  pall  upon  the  wheel  D  engages  the  ratchet  on  the 
down  stroke  of  the  piston  only,  on  the  up  stroke  it  slips  freely 
past  the  ratchet.  The  piston  A  is  therefore  quite  free  to  move 
without  the  shaft  on  the  up  stroke,  but  it  engages  on  the  down 
stroke.  The  cams  F  and  G  are  arranged  to  strike  projections 
upon  the  rack,  and  so  raise  or  lower  the  piston.  It  is  raised  when 
the  charge  is  to  be  taken  in,  and  lowered  when  it  has  completed 
its  working  stroke  and  the  exhaust  gases  have  to  be  discharged. 
When  raised  the  valve  H  is  in  the  position  shown.  Air  first  enters 
the  cylinder  through  the  port  i,  which  also  serves  to  discharge  the 
exhaust.  After  the  piston  has  uncovered  the  port  K  the  valve  H 
shuts  on  i,  opening  at  the  same  time  on  K  ;  the  gas  supply  then 
enters  and  mixes  more  or  less  perfectly  with  the  air  previously 
introduced. 


12  The  Gas  Engine 

A  small  further  movement  of  the  piston  now  closes  the  valve, 
and  the  explosion  is  caused  by  the  passage  of  the  electric  spark  in 
the  position  indicated  upon  the  drawing.  The  piston  shoots  up 


;/ 
FIG.  6. — Barsanti  and  Matteucci  Engine,  1857. 

freely  to  the  top  of  its  stroke,  to  give  out  the  work  stored  up  usefully 
upon  its  return. 

As  the  next  engine  to  be  described  marks  the  beginning  of  the 


Historical  Sketch  of  the  Gas  Engine  1 3 

practicable  stage  of  gas  engine  development,  it  is  advisable  to 
summarise  before  proceeding. 

Previous  to  1860  the  gas  engine  was  entirely  in  the  experimental 
stage.  Many  attempts  were  made,  but  none  of  the  inventors 
sufficiently  overcame  the  practical  difficulties  to  make  any  of  their 
engines  commercially  successful.  This  was  mostly  due  to  the 
very  serious  nature  of  the  difficulties  themselves,  but  it  was  also 
due  to  too  great  ambition  of  the  inventors  ;  they  wished  not  only 
to  compete  with  the  steam  engine  for  small  powers,  but  for  large 
powers.  They  thought  in  fact  more  to  displace  the  steam  engine 
than  to  compete  with  it. 

This  is  clearly  shown  in  many  of  their  descriptions  of  the  appli- 
cations of  their  inventions. 

The  greatest  credit  is  due  to  Wright  and  Barnett.  Wright 
very  closely  proposed  the  modern  non-compression  system,  Barnett 
the  modern  compression  system.  Barnett  is  also  the  originator  of 
one  of  the  modern  flame  systems  for  ignition.  Barsanti  and 
Matteucci  follow  in  order  of  merit  as  the  inventors  of  the  free-piston 
gas  engine. 

M.  Lenoir  occupies  the  honourable  position  of  the  inventor 
of  the  first  gas  engine  ever  actually  introduced  to  public  use.  The 
engine  was  not  strikingly  novel ;  nothing  was  done  in  it  which  had 
not  been  proposed  before,  but  its  details  were  thoroughly  and 
carefully  worked  out.  It  was  in  fact  the  first  to  emerge  from  the 
purely  experimental  stage.  Lenoir's  real  credit  consists  in  over- 
coming the  practical  difficulties  sufficiently  to  make  previous 
proposals  fairly  workable. 

The  principle  is  exceedingly  simple  and  evident.  The  piston 
moves  forward  for  a  portion  of  its  stroke,  by  the  energy  stored  in 
the  fly  wheel,  and  takes  into  the  cylinder  a  charge  of  gas  and  air 
at  the  ordinary  atmospheric  pressure.  The  valves  cut  off  com- 
munication, and  the  explosion  is  occasioned  by  the  electric  spark  ; 
this  propels  the  piston  to  the  end  of  the  stroke.  Exhausting  is 
done  precisely  as  in  the  steam  engine. 

The  engine  is  simply  an  ordinary  high-pressure  steam  engine  with 
valves  arranged  to  admit  gas  and  air  and  discharge  the  products 
of  combustion.  Fig.  7  is  an  external  elevation  of  a  three-horse 
engine.  It  was  first  constructed  in  Paris  in  1860  by  M.  Hippolyte 


The  Gas  Engine 


Historical  Sketch  of  the  Gas  Engine  1 5 

Marinoni.  In  Moigno's  'Cosmos'  of  that  year  it  is  stated  that  two 
engines  were  in  course  of  manufacture,  one  of  six  horse-power,  the 
other  of  twenty. 

The  early  statements  of  its  economy  were  ludicrously  inaccurate. 
A  one  horse-power  engine  consumed,  it  was  said,  but  3  cubic  metres 
(106  cubic  ft.  nearly)  of  coal  gas  in  twelve  hours'  work,  and  therefore 
cost  for  fuel  not  more  than  one-half  of  what  a  steam  engine  would 
have  done. 

The  actual  consumption  was  speedily  shown  to  be  much  nearer 
3  cubic  metres  per  effective  horse-power  per  hour. 

Notwithstanding  the  high  consumption,  the  engine  had  many 
good  points  ;  its  action  was  exceedingly  smooth  ;  no  shock  whatever 
was  heard  from  the  explosion.  Indeed  it  is  quite  impossible  when 
watching  the  engine  in  motion  to  realise  that  regular  explosions 
are  occurring.  The  motion  is  as  smooth  and  silent  as  in  the  best 
steam  engines. 

In  the  '  Practical  Mechanics'  Journal'  of  August  1865,  there 
is  an  article  describing  the  progress  made  by  the  engine  since  the 
date  of  its  introduction,  from  which  it  appears  that  in  Paris  and 
France  from  300  to  400  engines  were  then  at  work,  the  power  ranging 
from  half  horse  to  three  horse. 

The  Reading  Iron  Works  Company,  Limited,  at  Reading,  un- 
dertook the  manufacture  for  this  country.  One  hundred  engines 
were  made  and  delivered  by  them  :  several  of  them  have  continued 
at  work  till  now.  Notably  one  engine  inspected  by  the  author  at 
Petworth  House,  Petworth,  worked  for  twenty  years  pumping  water, 
and  is  even  yet  in  good  condition. 

The  work  performed  by  the  engines  was  multifarious  in  its 
character — printing,  pumping  water,  driving  lathes,  cutting  chaff, 
sawing  stone,  polishing  marble,  in  fact,  wherever  from  one-half  to 
three  horse-power  was  sufficient. 

Lenoir's  patent  in  this  country  was  obtained  by  J.  H.  Johnson, 

1860,  No.  335.     It   describes  very  closely  the  engine  as  manu- 
factured both  in  France  and  England.     The  subsequent  patent, 

1861,  No.  107,  does  not  seem  to  have  been  carried  into  effect. 
These  specifications  contain  many  erroneous  ideas,  showing 

the   notions  then  prevalent   among   inventors  of  the  nature   of 
gaseous  explosions.    Lenoir  erroneously  supposed  that  the  economy 


1 6  The  Gas  Engine 

of  his  engine  would  be  improved  if  he  could  obtain  a  slower 
explosion.  He  evidently  thought  that  the  power  imparted  to  the 
piston  by  explosion  was  similar  in  nature  to  a  sudden  blow — a 
rapid  rise  of  pressure,  and  a  fall  nearly  as  rapid  He  therefore 
attempted  to  avoid  explosion  by  such  expedients  as  stratification 
and  injection  of  steam  or  water  spray.  The  stratification  idea  he 
very  clearly  expressed  in  his  second  specification,  stating  that  '  the 
object  of  preventing  the  admixture  of  air  and  gas  is  to  avoid 
explosion.'  It  is  somewhat  extraordinary  to  find  notions  so 
erroneous  common  at  a  time  when  Bunsen's  work  had  clearly 
proved  the  continuous  nature  of  the  combustion  in  gaseous  explo- 
sions, and  when  Hirn  had  made  experiments  which  shov.-ed  that 
the  heat  evolved  by  explosion  in  a  gas  engine  was  only  a  small 
part  of  the  total  heat  of  the  combustion,  the  heat  which  did  not 
appear  during  explosion  being  produced  during  expansion. 

Other  speculations  on  the  cause  of  the  uneconomical  working 
of  the  engine  were  frequent,  but  the  true  reason  was  fully  explained 
by  Gustav  Schmidt  in  a  paper  read  before  '  The  Society  of  Ger- 
man Engineers'  in  1861.  He  states  :  'The  results  would  be  far 
more  favourable  if  compression  pumps,  worked  from  the  engine, 
compressed  the  cold  air  and  cold  gas  to  three  atmospheres  before 
entrance  into  the  cylinder  ;  by  this  a  greater  expansion  and  trans- 
formation of  heat  is  possible.' 

This  opinion  became  common  at  this  time.  Compression 
engines  were  proposed  with  great  clearness  and  a  full  understand- 
ing of  the  advantages  to  be  gained. 

Million,  1 86 1,  No.  1840. — This  Frenchman  had  exceedingly 
clear  ideas  of  the  advantages  of  compression  ;  he  evidently  con- 
siders himself  as  the  first  to  propose  its  use  in  a  gas  engine, 
apparently  unaware  of  the  existence  of  Barnett's  engine  already 
described.  He  claims  the  exclusive  right  to  use  compression  in 
the  most  emphatic  language. 

The  first  engine  described  is  exactly  what  Schmidt  asks  for. 
Separate  pumps  compress  the  air  and  gas  into  a  reservoir,  from 
which  the  movement  of  the  motor  piston,  during  a  portion  of  the 
stroke,  withdraws  its  charge  under  compression.  Ignition  is  ac- 
complished by  the  electric  spark,  and  the  piston  moves  forward 
under  the  high  pressure  produced.  He  states : 


Historical  Sketch  of  tlie  Gas  Engine  1 7 

'  In  ordinary  air  engines  the  operation  of  the  motive  cylinders 
is  analogous  to  that  of  the  pumps,  the  result  being  that  there  are 
two  cylinders,  which  act  in  directions  contrary  to  each  other,  and 
that  the  pump,  which  is  an  organ  of  resistance,  even  works  at  a 
greater  pressure  than  that  of  the  motive  cylinder,  which  is  an 
organ  of  power.  Thus  these  engines  are  very  large  in  proportion  to 
their  power.  On  the  contrary  by  employing  gases  under  the  con- 
ditions above  explained,  these  engines  will  exert  great  power  in 
proportion  to  their  dimensions.  The  sudden  ignition  of  the  gases 
in  the  motive  cylinder  causes  the  latter  to  work  at  an  operative 
pressure  much  greater  than  that  of  the  pumps.' 

The  advantage  of  compression  in  a  gas  engine  could  not  be 
more  fully  and  clearly  stated.  But  he  goes  even  a  step  further  ; 
he  sees  that  the  portion  of  the  motor  piston  stroke  spent  in  taking 
in  the  charge  under  compression,  is  a  disadvantage,  and  he  pro- 
poses to  make  the  whole  stroke  available  for  power  by  providing  a 
space  at  the  end  of  the  cylinder  in  which  the  gases  are  compressed. 

'Instead  of  introducing  the  cold  gases  into  the  cylinders, 
during  a  portion  of  the  stroke  and  igniting  them  afterwards,  when 
the  induction  ceases  .  .  .  another  arrangement  might  be  adopted. 
The  motive  cylinder  might  be  made  longer  than  necessary,  in 
order  that  the  piston  should  always  leave  between  it  and  the  end 
of  the  cylinder  a  greater  or  less  space,  according  to  the  pleasure 
of  the  constructor,  such  as  one-fourth  or  one-third,  more  or  less, 
of  the  volume  generated  by  the  motive  piston.  This  space  is 
called  by  the  inventor  a  cartridge.  On  opening  the  slide  valve 
the  gases  could  be  allowed  to  enter  suddenly  from  the  pressure 
reservoir  into  this  cartridge  towards  the  dead  point,  and  this  induc- 
tion having  ceased,  an  electric  spark  would  ignite  the  gases  in 
the  cartridge  by  which  the  driving  piston  would  be  set  in  motion.' 

Such  an  engine  would  resemble  in  its  action  the  best  modern 
compression  engines.  The  difficulties  of  ignition  however  are  too 
considerable  to  be  overcome  without  further  detail. 

The  compression  idea  at  this  date  was  evidently  widely 
spread,  because  it  again  crops  up  in  a  remarkably  clever  pamphlet 
by  M.  Alph.  Beau  de  Rochas,  published  at  Paris  in  1862.  He 
advances  a  step  further  than  Million,  and  investigates  the  con- 
ditions of  greatest  economy  in  gas  engines  using  compression, 

c 


1 8  TJ-ie  Gas  Engine 

with  reference  to  volume  of  hot  gases  and  surfaces  exposed.  He 
states  that  to  obtain  economy  with  an  explosion  engine,  four  con- 
ditions are  requisite  : 

1.  The  greatest  possible  cylinder  volume  with  the  least  pos- 
sible cooling  surface. 

2.  The  greatest  possible  rapidity  of  expansion. 

3.  The  greatest  possible  expansion  ;  and 

4.  The  greatest  possible  pressure  at  the  commencement  of  the 
expansion. 

In  using  boiler  tubes,  he  states,  the  efficiency  of  the  heat 
transmitted  increases  with  reduction  in  the  diameter  of  the  tubes. 
In  the  case  of  engine  cylinders,  therefore,  the  loss  of  heat  of  explo- 
sion would  be  in  inverse  ratio  to  the  diameter  of  the  cylinders. 

Therefore,  he  reasons,  an  arrangement  which  for  a  given  con- 
sumption of  gas,  gives  cylinders  of  the  greatest  diameters,  will 
give  the  best  economy,  or  least  loss  of  heat  to  the  cylinder.  One 
cylinder  only  must  be  employed  in  such  an  engine. 

But  loss  of  heat  depends  also  upon  time  ;  cooling,  therefore, 
will  be  proportionately  greater  as  the  working  speed  is  slower. 

Thesole  arrangement  capable  of  combining  these  conditions,  he 
states,  consists  in  using  the  largest  possible  cylinder,  and  reducing 
the  resistance  of  the  gases  to  a  minimum.  This  leads,  he  states, 
to  the  following  series  of  operations. 

1.  Suction  during  an  entire  outstroke  of  the  piston. 

2.  Compression  during  the  following  instroke. 

3.  Ignition  at  the  dead  point  and  expansion  during  the  third 
stroke. 

4.  Forcing  out  of  the  burned  gases  from  the  cylinder  on  the 
fourth  and  last  return  stroke. 

The  ignition  he  proposes  to  accomplish  by  the  increase  of 
temperature  due  to  compression.  This  he  expects  to  do  by 
compressing  to  one- fourth  of  the  original  volume. 

In  our  own  country  the  late  Sir  C.  W.  Siemens  proposed  conr 
pression  in  1862.  The  idea  was  exceedingly  widely  spread,  as  is 
evident  from  those  numerous  and  independent  inventions.  The 
practical  experience  to  enable  it  to  be  successfully  effected  had 
yet  to  be  created,  however,  and  this  took  many  years  of  patient 
work. 


Historical  Sketch  of  the  Gas  Engine  19 

The  igniting  arrangement  was  the  first  weak  point  requiring 
improvement.  The  electrical  method  of  Lenoir  was  exceedingly 
delicate  and  troublesome. 


C2 


2O  TJie  Gas  Engine 

Hugon's  engine,  produced  in  1865,  was  similar  to  Lenoir's  ; 
but  the  igniting  was  accomplished  by  flame,  a  modification  of 
Barnett's,  1838,  using  a  slide  valve  instead  of  a  lighting  cock. 
The  flame  ignition  was  certain  and  easily  kept  in  order.  In 
other  points  the  engine  was  a  great  improvement  upon  its  prede- 
cessor. The  lubrication  was  improved  by  injecting  water  into  the 
cylinder  and  the  cooling  water  jacket  was  better  arranged.  As  a 
result  the  consumption  of  gas  was  reduced. 

Fig.  8  is  an  external  elevation  of  the  Hugon  engine. 

Mr.  Otto  now  appears  upon  the  scene.  Before  him  much  had 
been  done  in  inventing  and  studying  engines,  but  it  remained  for 
him  by  sheer  perseverance  and  determination  of  character,  to 
overcome  all  difficulties  and  reduce  to  successful  practice  the 
theories  of  his  predecessors. 

In  1867  Messrs.  Otto  and  Langen  exhibited  at  the  Paris  exhibi- 
tion of  that  year,  their  free  piston  engine,  exterior  elevation  shown 
at  fig.  9.  It  was  absolutely  identical  in  principle  with  the  previous 
invention  of  Barsanti  and  Matteucci,  but  the  details  were  com- 
pletely and  successfully  carried  out.  The  Germans  succeeded 
commercially  and  scientifically  when  the  Italians  completely 
failed. 

Flame  ignition  was  used  and  great  economy  was  obtained,  a  half- 
horse  engine,  according  to  Professor  Tresca,  giving  over  half-horse 
power  effective,  on  a  gas  consumption  at  the  rate  of  44  cubic  feet 
per  effective  horse-power  per  hour.  This  is  less  than  half  the 
consumption  of  Lenoir  or  Hugon  ;  accordingly  the  prejudice  ex- 
cited by  the  strange  appearance  and  noisy  action  of  the  engine 
did  not  prevent  its  sale  in  large  numbers.  It  completely  crushed 
Lenoir  and  Hugon,  and  held  almost  sole  command  of  the  market 
for  ten  years,  several  thousands  being  constructed  in  that  period. 

The  Brayton  gas  engine  appeared  in  America  in  1873,  but 
although  more  mechanical  than  any  free  piston  engine,  its  economy 
was  insufficient  to  enable  it  to  compete.  It  was  better  than  Lenoir 
or  Hugon,  but  not  nearly  so  good  as  Otto  and  Langen. 

Other  inventors  attempted  free  piston  engines,  but  with  small 
success. 

In  1876  Mr.  Otto  superseded  his  former  invention  by  the 
production  of  the  '  Otto  Silent '  engine,  now  known  all  over  the 


Historical  Sketch  of  the  Gas  Engine 


21 


globe.  It  is  a  compression  engine,  using  the  precise  cycle  described 
in  1862  by  Beau  de  Rochas,  but  carried  out  in  a  most  perfect 
manner  and  using  a  good  form  of  flame  ignition,  a  modified  Otto  and 
Langen  valve  in  fact.  The  economy  is  greater  than  that  of  any 


FlG.  9. — Otto  and  Langen  Free  Piston  Engine. 

previous  engine,  one  indicated  horse  being  obtained  upon  20  cubic 
feet  of  gas,  or  one  effective  horse  upon  24  to  30  cubic  feet  per 
hour. 

This  engine  has  established  gas  engines  upon  a  firm  commercial 
basis,  15,000  having  been  sold  since  its  invention  ;  this  represents 
at  least  an  effective  power  of  90,000  horses. 


22  The  Gas  Engine 

Strangely  enough,  although  Mr.  Otto  is  the  greatest  and  most 
successful  gas  engine  inventor  who  has  yet  appeared,  he  adheres 
to  Lenoir's  erroneous  ideas,  and  in  his  specification  2081  of  1876 
he  attributes  the  economy  of  his  machine  to  a  slow  explosion  caused 
by  arrangement  of  gases  within  the  cylinder. 

The  compression,  which  is  the  real  cause  of  the  economy  and 
efficiency  of  the  machine,  he  seems  to  consider  as  an  accidental  and 
unessential  feature  of  his  invention. 

The  gas  engine,  like  all  great  inventions,  is  the  result  of  the 
long-continued  labour  of  many  minds  ;  it  is  a  gradual  growth  due 
to  the  united  labours  of  many  inventors.  In  the  earlier  days  of 
motive  power,  explosion  was  as  much  in  the  minds  of  the  inventors, 
Huyghen.s  and  Papin,  as  steam,  but  the  mechanical  difficulties 
proved  too  great.  The  constructive  skill  of  the  time  was  heavily 
taxed  by  the  rude  steam  engine  of  Newcomen,  and  still  more 
unequal  to  the  invention  of  James  Watt  ;  it  was  in  1774  that  Watt 
ran  his  first  successful  steam  engine  at  Soho  Works,  Birmingham. 
Twenty  years  later,  1794,  Street's  gas  engine  patent  indicated  the 
direction  of  men's  minds,  seeking  a  rival  for  steam  before  steam  had 
been  completely  introduced.  The  experience  and  skill  accumulating 
in  the  construction  of  the  steam  engine  made  the  gas  engine  more 
and  more  possible. 

The  proposals  of  Brown,  1823  ;  Wright,  1833  ;  Barnett,  1838  ; 
Barsanti  and  Matteucci,  1857,  show  gradually  increasing  knowledge 
of  detail  and  the  difficulties  to  be  overcome,  all  leading  to  the  first 
practicable  engine  in  1860,  the  Lenoir. 

Since  that  date  till  now,  twenty-five  years,  great  advances  have 
been  made,  and  at  present  the  gas  engine  is  the  only  real  rival  to 
steam. 


CHAPTER  I. 

THE    GAS    ENGINE    METHOD. 

GAS  ENGINES,  while  differing  widely  in  theory  of  action  and 
mechanical  construction,  possess  one  feature  in  common  which 
distinguishes  them  from  other  heat  engines  :  that  feature  is  the 
method  of  heating  the  working  fluid. 

The  working  fluid  is  atmospheric  air,  and  the  fuel  required  to 
heat  it  is  inflammable  gas.  In  all  gas  engines  yet  produced,  the 
air  and  gas  are  mixed  intimately  with  each  other  before  introduc- 
tion to  the  motive  cylinder  ;  that  is,  the  working  fluid  and  the 
iuel  to  supply  it  with  heat  are  mixed  with  each  other  before  the 
combustion  of  the  fuel. 

The  fuel,  which,  in  the  steam  and  in  most  hot-air  engines,  is 
burned  in  a  separate  furnace,  is,  in  the  gas  engine,  introduced 
directly  to  the  motive  cylinder  and  burned  there.  It  is  indeed 
part  of  the  working  fluid. 

This  method  of  heating  may  be  called  the  gas-engine  method, 
and  from  it  arises  at  once  the  great  advantages  and  also  the  great 
difficulties  of  these  motors. 

Compare  first  with  the  steam  engine.  In  it  there  exist  two 
great  causes  of  loss  :  water  is  converted  into  steam,  absorbing  a 
great  amount  of  heat  in  passing  from  the  liquid  to  the  gaseous 
state  ;  after  it  has  been  used  in  the  engine  it  is  rejected  into  the 
atmosphere  or  the  condenser,  still  existing  as  steam.  The  heat 
necessary  to  convert  it  from  the  liquid  to  the  gas  is  consequently 
in  most  part  rejected  with  it.  Loss,  occurring  in  this  way,  would 
be  small  if  high  temperatures  could  be  used  ;  but  this  is  the  point 
where  steam  fails.  High  temperatures  cannot  be  obtained  without 
pressure  so  great  as  to  be  quite  unmanageable.  The  attempt  to  obtain 
high  temperatures  by  super-heating  has  often  been  made,  but  with- 


24  The  Gas  Engine 

out  any  substantial  success.  Although  the  difficulty  of  excessive 
pressure  is  avoided,  another  set  of  troubles  are  introduced.  All  the 
heat  to  be  given  to  the  gaseous  steam  must  pass  through  the  iron 
plates  forming  the  boiler  or  super-heater,  which  plates  will  only 
stand  a  comparatively  low  temperature,  certainly  not  exceed- 
ing that  of  a  low  red  heat,  or  about  600°  to  700°  C.  Steam, 
being  a  gas,  is  much  more  difficult  to  heat  than  water ;  it  follows 
that  even  these  temperatures  cannot  be  attained  without  enormous 
addition  to  the  heating  surface.  The  difficulties  of  making  a 
workable  engine  using  high  temperature  steam  are  so  great  that 
even  so  distinguished  an  engineer  and  physicist  as  the  late  Sir 
C.  W.  Siemens  failed  in  his  attempts,  which  extended  over  many 
years.  It  may  be  taken  then  that  low  temperature  is  the  natural 
and  unavoidable  accompaniment  of  the  steam  method,  arising 
from  the  necessary  change  of  the  physical  state  of  the  working 
fluid,  and  the  limited  temperature  which  iron  will  safely  bear. 
The  originators  of  the  science  of  thermodynamics  have  long 
taught  that  the  maximum  efficiency  of  a  heat  engine  is  obtained 
when  there  is  the  maximum  difference  between  the  highest  and 
lowest  temperatures  of  the  working  fluid.  So  long  ago  as  1854, 
Professor  Rankine  read  a  paper  before  the  British  Association, 
'  On  the  means  of  realising  the  advantages  of  the  Air  Engine,'  in 
which  he  expresses  his  belief  that  such  engines  will  be  found  to  be 
the  most  economical  means  of  developing  motive  power  by  the 
agency  of  heat.  In  this  opinion  he  stood  by  no  means  alone. 
Engineers  so  able  as  Stirling,  Ericsson,  and  Siemens  ;  physicists 
so  distinguished  as  Dr.  Joule,  and  Sir  Wm.  Thomson,  devoted 
much  energy  and  study  to  their  practice  and  theory.  Notwith- 
standing all  their  efforts,  aided  by  a  host  of  less  able  inventors, 
the  difficulties  proved  too  formidable  ;  and  although  more  than 
thirty  years  have  now  passed  since  Rankine  announced  his  belief, 
the  hot-air  engine  proper,  has  made  no  real  advance.  Similar 
causes  to  those  acting  in  the  steam  engine  impose  a  limit  here. 
It  is  true  the  complication  of  changing  physical  state  is  avoided, 
but  the  limited  resistance  of  iron  to  heat  acts  as  powerfully 
as  ever.  Air  is  much  more  difficult  to  heat  than  water,  and,  there- 
fore, requires  a  much  larger  surface  per  unit  of  heat  absorbed. 
In  the  larger  hot-air  engines,  accordingly,  the  furnaces  and  heating 


TJie  Gas  Engine  Method  2$ 

surfaces  gave  great  trouble.  Very  low  maximum  temperatures 
were  attained  in  practice.  In  a  Stirling  engine  giving  out  thirty- 
seven  brake  horse-power,  the  maximum  temperature  was  only 
343°  C.  ;  in  the  engines  of  the  ship  '  Ericsson,'  the  maximum  was 
only  about  212°  C.,  according  to  Rankine,  the  indicated  power 
being  about  300  horses.  These  figures  show  that  the  heating 
surfaces  were  insufficient,  as  in  both  cases  the  furnaces  were  pushed 
to  heat  the  metal  to  a  good  red.  A  method  of  internal  firing  was 
proposed,  first  by  Sir  George  Cayley  and  afterwards  carried  out 
with  some  success  by  others  ;  the  furnace  was  contained  in  a 
completely  closed  vessel,  and  the  air  to  be  heated  was  forced 
through  it  before  passing  to  the  motor  cylinder.  The  plan  gave 
better  results,  but  the  temperature  of  700°  C.  was  still  the  limit,  as 
the  strength  of  the  iron  reservoir  had  to  be  considered,  and  the 
hot  gases  had  to  pass  through  valves.  Wenham's  engine,  described 
in  a  paper  read  before  the  Institution  of  Mechanical  Engineers  in 
1873,  is  a  good  example  of  this  class.  In  it  the  highest  temperature 
of  the  working  fluid,  as  measured  by  a  pyrometer,  was  608°  C. ; 
higher  temperatures  could  easily  have  been  got  but  the  safety  of 
the  engine  did  not  permit  it.  Professor  Rankine  in  his  work  on 
the  steam  engine  has  very  fully  discussed  the  disadvantages  arising 
from  low  maximum  temperatures.  He  calculates  that  in  a  perfect 
air  engine  without  regenerator  an  average  pressure  of  8-3  Ibs. 
per  square  inch  would  only  be  attained  with  a  maximum  of 
216 -6  Ibs.  per  square  inch,  thus  necessitating  great  strength  of 
cylinder  and  working  parts  for  a  very  small  return  in  effective 
power.  In  the  '  Ericsson,'  the  average  effective  pressure  was  less 
than  this,  being  only  about  2  Ibs.  per  square  inch;  it  had  four  air 
cylinders  each  of  14  feet  diameter,  and  only  indicated  300  horse- 
power. Stirling's  motor  cylinder  did  not  give  a  true  idea  of  the 
bulk  of  the  engine,  as  the  real  air-displacer  was  separate.  Even 
with  Wenham's  machine  the  bulk  was  excessive,  an  engine  of 
24  inches  diameter  cylinder  and  12  inches  stroke  giving  4  horse- 
power. 

Those  facts  sufficiently  illustrate  the  practical  difficulties  which 
prevented  the  development  of  the  hot-air  engine  proper.  All  flow 
from  the  method  of  heating.  Low  temperature  is  necessary  to 
secure  durability  of  the  iron. 


26  The  Gas  Engine 

All  hot-air  engines  are,  therefore,  very  large  and  very  heavy 
for  the  power  they  are  capable  of  exerting. 

The  friction  of  the  parts  is  so  great  that  although  the  theoreti- 
cal efficiency  of  the  working  fluid  is  higher  than  in  the  best  steam 
engines,  the  practical  efficiency  or  result  per  horse  available  for 
external  work  is  not  nearly  so  great.  The  best  result  ever  claimed 
for  Stirling's  engine  is  27  Ibs.  of  coal  per  bk.  horce-power  per 
hour,  probably  under  the  truth,  but  even  allowing  it,  a  first  class 
steam  engine  of  to-day  will  do  much  better.  According  to  Prof. 
Norton,  the  engines  of  the  '  Ericsson  '  used  1-87  Ibs  of  anthracite 
per  indicated  horse-power  per  hour  ;  but  the  friction  must  have  been 
enormous.  Compared  with  the  steam  engine,  the  practical  disadvan- 
tages of  the  hot-air  engine  are  much  greater  than  its  advantage  of 
theory.  Owing  to  the  great  inferiority  of  air  to  boiling  water  as  a 
medium  for  the  convection  of  heat,  the  efficiency  of  the  furnace  is 
much  lower  ;  owing  to  the  high  maximum  and  low  available  pres- 
sure, the  friction  is  much  greater— which  disadvantages  in  practice 
more  than  extinguish  the  higher  theoretical  efficiency. 

The  gas  engine  method  of  heating  by  combustion  or  explosion 
at  once  disposes  of  those  troubles  ;  it  not  only  widens  the  limits 
of  the  temperatures  at  command  almost  indefinitely,  but  the  causes 
of  failure  with  the  old  method  become  the  very  causes  of  success 
with  the  new  method. 

The  difficulty  of  heating  even  the  greatest  masses  of  air  is 
quite  abolished.  The  rapidly  moving  flash  of  chemical  action 
makes  it  easy  to  heat  any  mass,  however  great,  in  a  minute  fraction 
of  a  second  ;  when  once  heated  the  comparatively  gradual  con- 
vection makes  the  cooling  a  very  slow  matter.  The  conductivity 
of  air  for  heat  is  but  slight,  and  both  losing  and  receiving  heat 
from  enclosing  walls  are  carried  on  by  the  process  of  convection, 
the  larger  the  mass  of  air  the  smaller  the  cooling  surface  relatively. 
Therefore  the  larger  the  volumes  of  air  used,  the  more  economical 
the  new  method,  the  more  difficult  the  old.  The  low  conductivity 
for  heat,  the  cause  of  great  trouble  in  hot-air  machines,  becomes 
the  unexpected  cause  of  economy  in  gas  engines.  If  air  were  a 
rapid  carrier  of  heat,  cold  cylinder  gas  engines  would  be  impos- 
sible. The  loss  to  the  sides  of  the  enclosing  cylinders  would  be 
so  great  that  but  little  useful  effect  could  be  obtained.  Even  as 


The  Gas  Engine  Method  27 

it  is,  present  loss  from  this  cause  is  sufficiently  heavy.  In  the 
earlier  engines  as  much  as  three-fourths  of  the  whole  heat  of  the 
combustion  was  lost  in  this  way  ;  in  the  best  modern  engines  so 
much  as  one-half  is  still  lost. 

A  little  consideration  of  what  is  occurring  in  the  gas  engine 
cylinder  at  each  explosion  will  show  that  this  is  not  surprising. 
Platinum,  the  most  infusible  of  metals,  melts  at  about  1700°  C.; 
the  ordinary  temperature  of  cast  iron  flowing  from  a  cupola  is 
about  1200°  C.;  a  temperature  very  usual  in  a  gas  engine  cylinder 
is  1600°  C.,  a  dazzling  white-heat.  The  whole  of  the  g-.ses  filling 
the  cylinder  are  at  this  high  temperature.  If  one  could  see  the 
interior  it  would  appear  to  be  filled  with  a  blinding  glare  of  light. 
This  experiment  the  writer  has  tried  by  means  of  a  small  aperture 
covered  with  a  heavy  glass  plate,  carefully  protected  from  the 
heat  of  the  explosion  by  a  long  cold  tube.  On  looking  through 
this  window  while  the  engine  is  at  work,  a  continuous  glare  of 
white  light  is  observed.  A  look  into  the  interior  of  a  boiler 
furnace  gives  a  good  notion  of  the  flame  filling  the  cylinder  of 
a  gas  engine. 

At  first  sight  it  seems  strange  that  such  temperature  can  be 
used  with  impunity  in  a  working  cylinder  ;  here  the  convenience 
of  the  method  becomes  evident.  The  heating  being  quite  inde- 
pendent of  the  temperature  of  the  walls  of  the  cylinder,  by  the  use 
of  a  water-jacket  they  can  be  kept  at  any  desired  temperature. 
The  same  property  of  rapid  convection  of  heat,  so  useful  for 
generating  steam  from  water,  is  essential  in  the  gas  engine  to  keep 
the  rubbing  surfaces  at  a  reasonable  working  temperature.  In 
this  there  is  no  difficulty,  and  notwithstanding  the  high  tempera- 
ture of  the  gases,  the  metal  itself  never  exceeds  the  boiling  point 
of  water. 

So  good  a  result  cannot  of  course  be  obtained  without  careful 
proportioning  of  the  cooling  surfaces  for  the  amount  of  heat  to  be 
carried  away  ;  in  all  modern  engines  this  is  carefully  attended  to, 
with  the  gratifying  result  that  the  cylinders  take  and  retain  a 
polished  surface  for  years  of  work  just  as  in  a  good  steam  engine. 

The  gas  engine  method  gives  the  advantage  of  higher  tempera- 
ture of  working  fluid  than  is  attainable  in  any  other  heat  engine, 
at  the  same  time  the  working  cylinder  metal  may  be  kept  as  cool 


28  Tfu  Gas  Engine 

as  in  the  steam  engine.  It  also  allows  of  any  desired  rate  of  heat- 
ing the  working  fluid  in  any  required  volumes. 

In  consequence  of  high  temperatures  the  available  pressures 
are  high,  and  therefore  the  bulk  of  the  engine  is  small  for  the 
power  obtained. 

It  realises  all  the  thermodynamic  advantages  claimed  for  the 
hot-air  engine  without  sacrificing  the  high  available  pressures  and 
rapid  rate  of  the  generation  of  power  which  is  the  characteristic  of 
the  steam  engine. 

For  rapid  convection  of  heat  existing  in  the  s'eam  boiler  is 
substituted  the  still  more  rapid  heating  by  explosion  or  combustion, 
a  rapidity  so  superior  that  the  power  is  generated  for  each  stroke 
separately  as  required,  there  being  no  necessity  to  collect  a  great 
magazine  of  energy. 

The  only  item  to  the  debtor  side  of  the  gas  engine  account 
is  the  flow  of  heat  through  the  cylinder  walls,  which  disadvantage 
is  far  more  than  paid  for  by  the  advantages. 


CHAPTER  II. 

GAS   ENGINES   CLASSIFIED. 

ALTHOUGH  the  gas  engine  patents  now  in  existence  number  many 
hundreds,  the  essential  differences  between  the  inventions  are  not 
great.  In  their  working  process  they  may  be  divided  into  a  few 
well-defined  types  : 

1.  Engines  igniting  at  constant  volume,  but  without  previous 
compression. 

2.  Engines  igniting  at  constant  pressure,  with  previous  com- 
pression. 

3.  Engines  igniting  at  constant  volume,  with  previous  com- 
pression. 

THE  FIRST  TYPE  is  the  simplest  in  idea  ;  it  is  the  most  apparent 
method  of  obtaining  power  from  an  explosion. 

In  it  the  engine  draws  into  its  cylinder  gas  and  air  at  atmos- 
pheric pressure,  for  a  part  of  its  stroke,  in  proportions  suitable  for 
explosion  ;  then  a  valve  closes  the  cylinder,  and  the  mixture  is 
ignited.  The  pressure  produced  pushes  forward  the  piston  for 
the  remainder  of  its  travel,  and  upon  the  return  stroke  the  pro- 
ducts of  the  combustion  are  expelled  exactly  as  the  exhaust  of  a 
steam  engine.  By  repeating  the  same  process  on  the  other  side 
of  the  piston,  a  kind  of  double-acting  engine  is  obtained.  It  is 
not  truly  double-acting,  as  the  motive  impulse  is  not  applied 
during  the  whole  stroke,  but  only  during  that  portion  of  it  left 
free  after  performing  the  necessary  function  of  charging  with  the 
explosive  mixture. 

The  working  cycle  of  the  engine  consists  of  four  operations  : 

1.  Charging  the  cylinder  with  explosive  mixture. 

2.  Exploding  the  charge. 

3.  Expanding  after  explosion. 

4.  Expelling  the  burned  gases. 


30  T/ie  Gas  Engine 

To  carry  it  out  in  a  perfect  manner,  the  mechanism  must  be 
so  arranged  that  during  the  charging,  the  pressure  of  the  gases  in 
the  cylinder  does  not  fall  below  atmosphere  ;  there  must  be  no 
throttling  of  the  entering  gases.  The  cut-off  and  the  explosion 
must  be  absolutely  simultaneous  and  also  instantaneous,  so  that 
the  heat  may  be  applied  without  change  of  volume,  and  thereby 
produce  the  highest  pressure  which  the  mixture  used  is  capable 
of  giving.  The  expansion  will  be  carried  fax  enough  to  reduce 
the  pressure  of  the  explosion  to  atmosphere  ;  and  the  exhaust 
stroke  will  be  accomplished  without  back  pressure.  The  charge 
in  entering  must  not  be  heated  by  the  walls  of  the  cylinder,  but 
should  remain  at  the  temperature  of  the  atmosphere  till  the  very 
moment  previous  to  ignition.  At  the  same  time,  the  cylinder 
should  not  cool  the  gases  after  the  explosion,  no  heat  should  dis- 
appear except  through  expansion  doing  work. 

Although  all  these  conditions  are  necessary  to  the  perfect 
cycle,  it  is  evident  that  no  actual  engine  is  capable  of  combining 
them.  Some  throttling  at  the  admission  of  the  mixture,  and  a 
little  back  pressure  during  the  exhausting  are  unavoidable  ;  some 
time  must  elapse  between  the  closing  of  the  inlet  valve  and  the 
explosion,  in  addition  to  the  time  taken  by  the  explosion  itself. 
Heat  will  be  communicated  to  the  entering  gases  and  lost  by  the 
exploded  gases  to  the  walls  of  the  cylinder. 

The  actual  diagram  taken  from  an  engine  will  therefore  differ 
considerably  from  the  theoretic  one. 

The  theoretical  conditions  are  to  a  great  extent  contradictory. 

The  idea  of  the  type,  however,  is  easily  comprehensible,  and 
evidently  suggested  by  the  common  knowledge  of  the  destructive 
effect  of  accidental  coal  gas  explosions  which  occurred  soon  after 
the  introduction  of  gas  into  general  use.  '  The  power  is  there,  let 
us  use  it  like  steam  in  the  cylinder  of  a  steam  engine/  said  the 
early  inventors. 

The  two  most  successful  engines  of  this  type  were  Lenoir's 
and,  later,  Hugon's,  for  very  small  powers  ranging  from  one  man 
to  half-horse.  Simple  forms  of  this  type  are  still  in  extensive  use. 
The  most  widely  known  of  these  is  the  Bisschof,  a  French  inven- 
tion. 

THE  SECOND  TVPE  is  not  so  simple  in  its  main  idea,  and  required 


Gas  Engines  Classified  3 1 

much  greater  knowledge  of  detail,  both  mechanical  and  theoretical. 
As  a  hot-air  engine  its  theory  was  originally  proposed  by  Sir  Geo. 
Cayley,  and,  later,  by  Dr.  Joule  and  Sir  Wm.  Thomson.  As  a 
hot-air  engine  it  failed  for  the  reasons  discussed  in  the  previous 
chapter. 

In  it  the  engine  is  provided  with  two  cylinders  of  unequal 
capacity  ;  the  smaller  serves  as  a  pump  for  receiving  the  charge 
and  compressing  it,  the  larger  is  the  motor  cylinder,  in  which  the 
charge  is  expanded  during  ignition  and  subsequent  to  it. 

The  pump  piston,  in  moving  forward,  takes  in  the  charge  at 
atmospheric  pressure,  in  returning  compresses  it  into  an  inter- 
mediate receiver,  from  which  it  passes  into  the  motor  cylinder  in 
a  compressed  state.  A  contrivance  similar  to  the  wire  gauze  in  a 
Davy  lamp  commands  the  passage  between  the  receiver  and  the 
cylinder,  and  permits  the  mixture  to  be  ignited  on  the  cylinder 
side  as  it  flows  in  without  the  flame  passing  back  into  the 
receiver. 

The  motor  cylinder  thus  receives  its  working  fluid  in  the  stale 
of  flame,  at  a  pressure  equal  to,  but  never  greater  than,  the 
pressure  of  compression.  At  the  proper  time,  the  valve  between 
the  motor  and  the  receiver  is  shut,  and  the  piston  expands  the 
ignited  gases  till  it  reaches  the  end  of  its  stroke,  when  the  exhaust 
valve  is  opened,  and  the  return  expels  the  burned  gases. 

The  ignition  here  does  not  increase  the  pressure,  but  increases 
the  volume.  The  pump,  say,  puts  one  volume  or  cubic  foot  into 
the  receiver  ;  the  flame  causes  it  to  expand  while  entering  the 
cylinder  to  two  cubic  feet.  It  does  the  work  of  two  cubic  feet  in 
the  motor  cylinder,  so  that,  though  there  is  no  increase  of  pres- 
sure, there  is  nevertheless  an  excess  of  power  over  that  spent  in 
compressing. 

In  the  first  type  of  engine  the  heat  is  given  to  the  working 
fluid  at  constant  volume,  in  the  second  type  the  heat  is  given  to 
the  working  fluid  at  constant  pressure  during  change  of  volume. 

The  working  cycle  of  the  engine  consists  of  five  operations  : 

1.  Charging  the  pump  cylinder  with  gas  and  air  mixture. 

2.  Compressing  the  charge  into  an  intermediate  receiver. 

3.  Admitting  the  charge  to  the  motor  cylinder  in  the  state  of 
flame,  at  the  pressure  of  compression. 


32  The  Gas  Engine 

4.  Expanding  after  admission. 

5.  Expelling  the  burned  gases. 

To  carry  out  the  process  perfectly  the  following  conditions 
would  be  required. 

No  throttling  during  admission  of  the  charge  to  the  pump. 

No  heating  of  the  charge  as  it  enters  the  pump  from  the 
atmosphere. 

No  loss  of  the  heat  of  compression  to  the  pump  and  receivei 
walls. 

No  throttling  as  the  charge  enters  the  motor  cylinder  from  the 
receiver. 

No  loss  of  heat  by  the  flame  to  the  sides  of  the  motor  cylinder 
and  piston. 

And  last,  No  back  pressure  during  the  exhaust  stroke. 

The  exhaust  gases  also  must  be  completely  expelled  by  the 
motor  piston  ;  that  is,  the  motor  cylinder  should  have  no  clear- 
ance. 

The  requirements  of  this  type,  although  sufficiently  numerous 
and  exacting,  are  not  so  contradictory  among  themselves  as  in  the 
first. 

Although  every  engine  of  the  kind  yet  made  fails  to  fulfil  them, 
it  is  quite  possible  that  a  machine  very  closely  approximating  may 
be  yet  constructed. 

The  most  successful  engines  of  this  kind  have  been  Brayton's 
and  Simon's,  the  first  an  American  invention,  and  the  second 
an  English  adaptation  of  it  Sir  C.  W.  Siemens  proposed  such 
an  engine  in  1861,  but  does  not  seem  to  have  been  successful  in 
carrying  it  out.  In  1860  it  was  also  proposed  by  F.  Million,  but 
without  a  sufficient  understanding  of  the  mechanical  detail  neces- 
sary for  a  working  machine. 

Brayton's  engine  was  made  in  considerable  numbers  in  America, 
and  was  applied  by  him  to  drive  a  good-sized  launch,  petroleum 
being  used  as  the  fuel  instead  of  gas.  It  was  exhibited  at  the  Cen- 
tennial Exhibition  in  Philadelphia  ;  at  the  Paris  exhibition  of  1878 
by  Simon. 

THE  THIRD  TYPE  is  the  best  kind  of  compression  engine  yet 
introduced  ;  by  far  the  largest  number  of  gas  engines  in  every  day 
use  throughout  the  world  are  made  in  accordance  with  its  require- 


Gas  Engines  Classified  33 

ments.  In  theory  it  is  more  easily  understood  as  requiring  two 
cylinders,  compression  and  po\ver. 

The  leading  idea,  compression  and  ignition  at  constant  volume, 
was  first  proposed  by  Barnett  in  1838,  then  by  Schmidt  in  more 
general  terms,  very  fully  by  Beau  de  Rochas  in  1860  and  also  by 
F.  Million  in  the  same  year.  Otto,  however,  was  the  first  to  suc- 
cessfully apply  it,  which  he  did  in  1876. 

The  compression  cylinder  may  be  supposed  to  take  in  the 
charge  of  gas  and  air  at  atmospheric  temperature  and  pressure  ; 
compress  it  into  a  receiver  from  which  the  motor  cylinder  is  sup- 
plied ;  the  motor  piston  to  take  in  its  charge  from  the  reservoir  in 
a  compressed  state  ;  and  then  communication  to  be  cut  off  and 
the  compressed  charge  ignited. 

Here  ignition  is  supposed  to  occur  at  constant  volume,  that  is, 
the  whole  volume  of  mixture  is  first  introduced  and  then  fired  ; 
the  pressure  therefore  increases.  The  power  is  obtained  by 
igniting  while  the  volume  remains  stationary  and  the  pressure  in- 
creases. 

Under  the  pressure  so  produced,  the  piston  completes  its 
stroke,  and  upon  the  return  stroke  the  products  of  the  combus- 
tion are  expelled 

In  this  case  the  working  cycle  of  the  engine  consists  of  six 
operations  : 

1.  Charging  the  pump  cylinder  with  gas  and  air  mixture. 

2.  Compressing  the  charge  into  an  intermediate  receiver. 

3.  Admitting  the   charge  to  the  motor  cylinder  under  com- 
pression. 

4.  Igniting  the  mixture  after  admission  to  the  motor. 

5.  Expanding  the  hot  gases  after  ignition. 

6.  Expelling  the  burned  gases. 

To  carry  out  the  process  perfectly,  similar  conditions  are  neces- 
sary to  those  in  the  second  type.  But  the  conditions  are  more 
contradictory.  The  gases  entering  the  cylinder  under  pressure  must 
not  be  heated  by  its  walls  ;  no  heat  should  be  added  till  the  igni- 
tion ;  then,  after  ignition  the  gases  must  not  lose  heat  to  the 
cylinder— conditions  which  it  is  impossible  for  the  same  cylinder 
to  fulfil  simultaneously. 

In  the  engines  constructed  the  receiver  is  dispensed  with,  for 

D 


34  The  Gas  Engine 

reasons  which  will  be  explained  in  discussing  the  practical  difficul- 
ties of  construction  ;  but  this  does  not  in  any  way  modify  the 
theory,  which  shall  first  be  discussed. 

The  most  considerably  used  engine  of  this  kind  is  the  Otto, 
next  to  it  coming  Clerk's  engine,  then  Robson's  by  the  Messrs. 
Tangye,  and  Andrews'  Stock  port  compression  engine.  In  none 
of  these  types  does  any  part  of  the  working  cycle  require  either 
the  heating  or  the  cooling  of  the  working  fluid  by  the  relatively 
slow  processes  of  convection  and  conduction. 

Heating  is  accomplished  by  the  rapid  method  of  explosion  or, 
if  the  term  be  preferred,  combustion,  and  for  the  cooling  neces-  • 
sary  in  all  heat  engines  is  substituted  the  complete  rejection  of 
the  working  fluid  with  the  heat  it  contains  and  its  replacement  by 
a  fresh  portion  taken  from  the  atmosphere  at  the  atmospheric 
temperature,  which  is  the  lower  limit  of  the  engines. 

This  is  the  reason  why  those  cycles  can  be  repeated  with 
almost  indefinite  rapidity,  and  why  gas  engines  can  be  run  at 
speeds  equal  to  steam  engines,  while  the  old  hot-air  engines  could 
not  be  run  fast,  because  of  the  very  slow  rate  at  which  air  could  be 
heated  and  cooled  by  contact. 

There  still  remains  one  important  type  of  gas  engine  not 
included  in  this  classification  ;  in  it  part  of  the  efficiency  is  de- 
pendent on  cooling  by  contact,  and  consequently  only  a  slow  rate 
of  working  stroke  can  be  obtained.  It  is  the  kind  of  engine 
known  as  the  free  piston  or  atmospheric  gas  engine.  It  may  be 
regarded  as  a  modification  of  the  first  type.  The  first  part  of  its 
action  is  precisely  similar,  the  latter  part  differs  considerably. 

It  may  be  called  Type  ONE  A.  In  it  the  piston  moves  for- 
ward, taking  in  its  charge  of  gas  and  air  from  the  atmosphere  at 
the  atmospheric  pressure  and  temperature.  When  cut  off  it  is 
ignited  instantaneously,  the  volume  being  constant  and  the 
pressure  increasing  :  the  piston  is  not  connected  directly  to  the 
motor  shaft,  but  is  free  to  move  under  the  pressure  of  the  explo- 
sion, like  the  ball  in  a  cannon.  It  is  shot  forward  in  the  cylinder 
(which  is  made  purposely  very  long)  ;  the  energy  of  the  explosion 
gives  the  piston  velocity  ;  it  therefore  continues  to  move  con- 
siderably after  the  pressure  has  fallen  by  expansion  to  atmos- 
phere ;  a  partial  vacuum  forms  under  the  piston  till  its  whole 


Gas  Engines  Classified  35 

energy  of  motion  is  absorbed  in  doing  work  upon  the  exterior 
air.  It  then  stops,  and  the  external  pressure  causes  it  to  perform 
its  instroke,  during  which  a  clutch  arrangement  yokes  it  to  the 
motor  shaft,  giving  the  shaft  an  impulse.  The  explosion  is  made 
to  give  its  equivalent  in  work  upon  the  external  air,  in  forming  a 
vacuum  in  fact ;  the  vacuum  is  increased  by  the  coo  ing  of  the 
hot  gases  during  the  return  of  the  piston.  The  piston  proceeds 
completely  to  the  boitom  of  the  cylinder,  expelling  the  products 
of  combustion.  So  far  as  the  working  fluid  of  the  engine  is  con- 
cerned the  cycle  consists  of  five  operations  : 

1.  Charging  the  cylinder  with  explosive  mixture. 

2.  Exploding  the  charge. 

3.  Expanding  after  explosion. 

4.  Compressing  the  burned  gases  after  some  cooling. 

5.  Expelling  the  burned  gases. 

To  carry  it  out  perfectly,  in  addition  to  the  requirements  of  the 
first  type,  the  expansion  should  be  carried  far  enough  to  lower  the 
temperature  of  the  working  fluid  to  the  temperature  of  the  atmos- 
phere, and  the  compression  to  atmospheric  pressure  again  should 
be  conducted  at  that  temperature  ;  that  is,  the  compression  line 
should  be  an  isothermal. 

This  kind  of  engine  was  proposed  first  by  Barsanti  and  Mat- 
teucci  in  1854,  by  F.  H.  Wenham  in  1864,  and  then  by  Otto  and 
Langen  in  1866.  The  last  named  inventors  were  successful  in 
overcoming  the  practical  difficulties,  and  many  engines  were 
made  and  sold  by  them.  Their  engine,  although  cumbrous  and 
noisy,  was  a  good  and  economical  worker  ;  many  are  still  in  use. 
The  next  best  known  engine  of  the  kind  was  Gillies's,  of  which  a 
considerable  number  were  constructed  and  sold. 


36  The  Gas  Engine 


CHAPTER   III. 

THERMODYNAMICS    OF   THE   GAS    ENGINE. 

BEGINNING  with  Professor  Rankine,  able  writers  have  so  fully 
treated  the  thermodynamics  of  the  air  engine  that  but  little  can 
be  added  to  the  knowledge  of  the  subject  now  in  existence.  The 
gas  engine  method  of  heating,  however,  introduces  limits  of 
temperature  so  extended  and  cycles  of  action  so  different  from 
those  possible  in  the  air  engine  proper,  that  something  remains  to 
be  done  in  applying  the  existing  data.  So  far  as  the  author  is 
aware,  this  has  been  previously  attempted  by  three  writers  only  — 
Prof.  R.  Schottler,  Dr.  A.  Witz,  and  himself. 

Before  proceeding  with  the  special  consideration  of  the  sub- 
ject, it  is  advisable  for  the  sake  of  completeness  to  state  briefly 
the  general  laws.  In  doing  so  Rankine  will  be  followed  as  closely 
as  possible. 

THERMODYNAMICS  DEFINED. 

'  It  is  a  matter  of  ordinary  observation  that  heat,  by  expanding 
bodies,  is  a  source  of  mechanical  energy,  and  conversely,  that 
mechanical  energy,  being  expended  either  in  compressing  bodies 
or  in  friction,  is  a  source  of  heat. 

'  The  reduction  of  the  laws  according  to  which  such  phenomena 
take  place  to  a  physical  theory  or  connected  system  of  principles 
constitutes  what  is  called  the  science  of  thermodynamics.' 

FIRST  LAW  OF  THERMODYNAMICS. 

Heat  and  mechanical  energy  are  mutually  convertible,  and 
heat  requires  for  its  production,  and  produces  by  its  disappear- 
ance, mechanical  energy  in  the  proportion  of  1,390  footpounds 
for  each  centigrade  heat  unit,  a  heat  unit  being  the  amount  of 


Thermodynamics  of  the  Gas  Engine  37 

heat  necessary  to  heat  one  pound  weight  of  water  through  i°  C. 
This  is  Joule's  law,  having  been  first  determined  by  him  in  1843. 
It  holds  with  equal  truth  for  other  forms  of  energy,  and  is  a 
general  statement  of  the  great  truth,  that  in  the  universe,  energy  is 
as  incapable  of  creation  or  destruction  as  matter.  Energy  may 
change  its  form  indefinitely  while  passing  from  a  higher  to  a 
lower  level,  but  it  can  neither  be  created  nor  destroyed.  The 
energy  of  outward  and  visible  movement  of  matter  may  be 
arrested  and  caused  to  disappear  as  movement  of  the  whole  mass 
in  one  direction,  but  its  equivalent  reappears  as  internal  move- 
ment or  agitation  of  the  particles  or  molecules  composing  the 
body.  Energy  assumes  many  forms,  but  the  sum  of  all  remains 
a  constant  quantity,  incapable  of  change  of  quantity,  but  capable 
of  disappearing  in  one  form  and  reappearing  in  another. 

SECOND  LAW  OF  THERMODYNAMICS. 

Although  heat  and  work  are  mutually  convertible  and  in 
definite  and  invariable  proportions,  yet  no  conceivable  heat 
engine  is  able  to  convert  all  the  heat  given  to  it  into  work. 

Apart  altogether  from  practical  limitations,  a  certain  portion 
~f  the  heat  must  be  passed  from  the  hot  body  to  the  cold  body  in 
order  that  the  remainder  may  assume  the  form  of  mechanical 
energy.  To  get  a  continuous  supply  of  mechanical  energy  from 
heat  depends  upon  getting  a  continuous  supply  of  hot  and  cold 
substances  :  it  is  by  the  alternate  expansion  and  contraction  of 
some  substance,  usually  steam  or  air,  that  heat  is  converted  into 
mechanical  energy. 

Perfect  heat  engines  are  ideal  conceptions  of  machines  which 
are  practically  impossible,  but  whose  operations  are  so  arranged 
that,  if  possible,  they  would  convert  the  greatest  conceivable 
proportion  of  the  heat  given  to  them  into  mechanical  work. 

Efficiency. — The  efficiency  of  a  heat  engine  is  the  ratio  of  the 
heat  converted  into  mechanical  work  to  the  total  amount  of  heat 
which  enters  the  engine. 

In  this  work  the  word  Efficiency,  when  used  without  qualifica- 
tion, bears  this  meaning  only. 

The   efficiency  of  a   perfect  heat  engine  depends  upon  two 


38  The  Gas  Engine 

things  alone  :  these  are,  the  temperature  of  the  source  of  heat  and 
the  temperature  of  the  source  of  cold  (allowing  the  expression). 
The  greater  the  difference  between  these  temperatures  the  greater 
the  efficiency.  That  is,  the  greater  will  be  the  proportion  of  the 
total  heat  converted  into  mechanical  energy,  and  the  smaller  the 
proportion  of  the  total  heat  which  necessarily  passes  by  conduction 
from  the  hot  to  the  cold  body. 

Properties  of  Gases.—  Gases  are  the  mo=t  suitable  bodies  for 
use  in  htat  engines  ;  they  are  almost  perfectly  elastic,  and  they  ex- 
pand largely  under  the  influence  of  heat. 

A  gas  is  said  to  be  perfect  when  it  completely  obeys  two 
laws  : 

1.  Boyle's  law. 

2.  Charles's  law. 

Boyle's  Law, — Suppose  unit  volume  of  gas  to  be  contained  in 
a  cylinder  fitted  with  a  piston  which  is  perfectly  tight  at  unit 
pressure.  Suppose  the  temperature  to  be  kept  perfectly  constant. 
Then,  according  to  Boyle's  law,  however  the  volume  may  be 
changed  by  moving  the  piston,  the  pressure  is  always  inversely 
proportional  to  volume,  that  is,  if  volume  becomes  two,  pressure 
becomes  one-half;  volume  becomes  three,  pressure  becomes 
one-third. 

The  product  of  pressure  and  volume  is  always  constant. 

Denoting  pressure  by/,  and  volume  by  r, 

Boyle's  law  is,  /  v  =  constant. 

Charles's  Law. — If  a  gas  kept  at  constant  volume  is  heated, 
the  pressure  increases.  If  a  gas  is  kept  behind  a  piston  which 
moves  without  friction  so  that  the  piessure  upon  the  gas  is  always 
consent,  the  heat  applied  will  cause  it  to  expand. 

One  volume  of  gas  at  o°  C.,  if  heated  through  i°  C.  will  expand 
.,{.,,  and  become  i^rj  volume,  if  the  pressure  is  constant.  If  the 
volume  is  constant,  then  its  pressure  will  increase  by  ¥13,  that  is, 
its  pressure  will  become  iiyi^  of  the  original.  In  the  same  way 
if  cooled  i°  C.  below  o°  C.,  it  will  contract  or  diminish  in  pressure 
h}"  L-7:i>  'ts  volume  or  pressure  becoming  fi|  of  what  it  is  at  o°  C. 

For  every  degree  of  heat  or  cold  above  or  below  o°  C.  a  perfect 
gas  expands  or  contracts  by  213  of  its  volume  at  o°  C. 


Thermodynamics  of  the  Gas  Engine  39 

From  this  it  is  evident  that  a  perfect  gas,  if  cooled  to  273°  C. 
below  o°  C.  will  have  neither  volume  nor  pressure. 

This  originally  gave  rise  to  the  conception  of  absolute  zero  of 
temperature.  The  absolute  temperature  of  a  body  is  ordinary 
temperature  Centigrade  +  273,  just  as  the  absolute  pressure  of  any 
gas  is  us  pressure  above  atmosphere  plus  atmospheric  pressure- .  The 
absolute  temperature  of  a  body  is  its  temperature  above  Centigrade 
zero  +  273. 

The  pressure  or  volume  of  a  gas  is  therefore  directly  propor- 
tional to  its  absolute  temperature. 

If /=pressure  for  absolute  temperature  /,  and  p^  pressure  for 
/'  temperature,  also  absolute, 

then        £=£,; 

or  if  v  be  the  volume  at  absolute  temperature  /  and  vl  at  /', 

then         ^  =  * 
vl       (* 

The  Second  Law  (quantitative). — If  heat  be  supplied  to  a  perfect 
heat  engine  at  the  absolute  temperature  x1,  and  the  absolute  tem- 
perature of  the  source  of  cold  is  T,  then  the  efficiency  of  that 
engine  is,  denoting  it  by  E, 

E  =  -  ~~—  =  i  -  -.. 

T1  T1 

It  is  unity  minus  the  lower  temperature  divided  by  the  upper 

T* 

temperature.     The  efficiency  is  greater  or  less  as  the  fraction    -( 

is  less  or  greater.  This  fraction  may  be  diminished  either  by 
reducing  T  or  by  increasing  T1.  The  lowest  available  temperature 
is  not  capable  of  great  variation,  being  in  our  climate  about  290" 
absolute.  It  therefore  follows  that  efficiency  could  only  be  in- 
creased by  increasing  T1. 

Suppose  x=29o°  absolute  and  T1  =  58o°  absolute. 

Then         E  =r  I  -  fffr  «  I  - 1  =  0-5. 

Suppose  T=290°,  and  T1  =  i45o°,  a  temperature  common  in 
gas  engines,  then 

p.  —    T   2  n  o    —  T 1    —  n-Q 

E    —     I    —  —   I     ~  00. 


The  Gas  Engine 


The  efficiency  increases  with  increase  of  the  maximum  tempe- 
rature. The  second  law,  in  its  quantitative  form,  is  the  statement 
of  the  efficiency  of  any  perfect  heat  engine  in  terms  of  absolute 
temperatures  of  the  source  of  heat  and  the  source  of  cold. 

Thermal  Lines. — If  a  volume  of  air  is  contained  in  a  cylinder 
having  a  piston  and  fitted  with  an  indicator,  the  piston,  if  moved 
to  and  fro,  will  alternately  compress  and  expand  the  air,  and  the 
indicator  pencil  will  trace  a  line  or  lines  upon  the  card,  which  lines 
register  the  change  of  pressure  and  volume  occurring  in  the  cylinder. 
If  the  piston  is  perfectly  free  from  leakage,  and  it  be  supposed  that 
the  temperature  of  the  air  is  kept  quite  constant,  then  the  line  so 
traced  is  called  an  Isothermal  Sine,  and  the  pressure  at  any  point 
when  multiplied  by  the  volume  is  a  constant  according  to  Boyle's 
law, 

pv  =  a  constant. 

If,  however,  the  piston  is  moved  in  very  rapidly,  the  air  will  not 
remain  at  constant  temperature,  but  the  temperature  will  increase 
because  work  has  been  done  upon  the  air,  and  the  heat  has  no 


r 

:,. 

.'•. 

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.c       7° 

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£       4° 

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( 

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<            10 

Atmospheric  line 


0      10     20     30    4°     50    60     70    80    90  ico 

VOLUME. 
Compression  lines  for  air  (dry),  Adiabatic  and  Isothermal. 

FIG.  10. 

time  to  escape  by  conduction.  If  no  heat  whatever  is  lost  by  any 
cause,  the  line  will  be  traced  over  and  over  again  by  the  indicator 
I  encil,  the  cooling  by  expansion  doing  work  precisely  equalling 
the  healing  by  compression.  This  is  the  line  of  no  transmission 
of  heat,  therefore,  known  as  Adiabafic.  Fig.  10  shows  these  two 


Tliermodynamics  of  the  Gas  Engine  4 1 

lines   for   air   starting  from  atmospheric  pressure  and   tempera- 
ture. 

The  pressures  at  different  points  of  the  curve  are  related  by  the 
equation 

pvy  =  constant. 

The  pressure  when  multiplied  by  the  volume  raised  to  the_y  power 
is  always  constant. 

The  power  _y  is  the  ratio  between  the  specific  heat  of  the  air  at 
constant  pressure  and  its  specific  heat  at  constant  volume.  Ac- 
cording to  Rankine 

y  =  1*408  for  air. 

Imperfect  Heat  Engines. — For  a  complete  description  of  the 
working  cycle  of  perfect  heat  engines,  the  reader  is  referred  to 
works  upon  the  steam  engine,  which  contain  the  fullest  possible 
details  both  of  reasoning  and  results. 

The  working  cycles  of  practicable  heat  engines  are  always  im- 
perfect, that  is,  the  operations  are  such  that,  although  perfectly 
carried  out,  the  maximum  efficiency  possible  by  the  second  law  of 
thermodynamics  could  not  be  attained  by  them.  Each  cycle  has 
a  maximum  efficiency  peculiar  to  itself,  which  is  invariably  less 

•"1*1     -—    T* 

than  -  — —  ,  but  which  does  not  necessarily  vary  with  T1  and  T. 

It  does  not  always  follow  that  increase  of  the  higher  tempera- 
ture causes  increase  of  efficiency  ;  conversely,  it  does  not  always 
follow  that  diminution  of  the  upper  temperature  causes  diminution 
of  efficiency.  Under  some  circumstances,  indeed,  the  opposite 
effect  is  produced— increase  of  the  upper  temperature  diminishes 
efficiency,  while  its  diminution  increases  it,  of  course  within  certain 
limits. 

All  the  gas  engine  cycles  described  in  the  previous  chapter  are 
imperfect  in  this  sense,  but  all  are  practicable.  It  follows  that  if 
any  one  of  them  gives  a  higher  efficiency  than  another  in  theory, 
it  will  also  do  so  in  practice,  provided  the  practical  losses  do  not  in- 
crease with  improved  theory. 

It  is  necessary  before  discussing  the  practical  losses  to  see  how 
the  cycles  compare  with  each  other,  if  each  be  perfectly  carried 
out.  The  results  obtained  can  then  be  modified  by  examination 
of  the  way  in  which  unavoidable  practical  losses  affect  each  cycle. 


42  The  Gas  Engine 


EFFICIENCY  FORMULA. 

If  H  is  the  quantity  of  heat  given  to  an  engine,  and  H1  the 
amount  of  heat  discharged  by  it  after  performing  work,  then,  the 
portion  which  has  disappeared  in  performing  work  is  H  —  H1, 
supposing  no  loss  of  heat  by  conduction  or  other  cause,  and  the 
efficiency  of  the  engine  is 


Type  i.  —  A  perfect  indicator  diagram  of  an  engine  of  this  kind 
is  shown  at  fig.  n:  the  line  abc  is  the  atmospheric  line,  represent- 
ing volume  swept  by  the  piston,  the  line  a  d  is  the  line  of  pressures. 
From  a  to  b  the  piston  moves  forward,  taking  in  its  charge,  at 
atmospheric  temperature  and  pressure;  at  b  communication  is  in- 
stantaneously cut  off,  and  heat  instantaneously  supplied,  raising 
the  temperature  to  the  maximum,  before  the  movement  of  the  pis- 
ton has  time  to  change  the  volume.  From  e,  the  point  of  maxi- 
mum temperature  and  pressure,  the  gases  expand  without  loss  of 
heat,  the  temperature  only  falling  by  reason  of  work  performed 
till  the  pressure  again  reaches  atmosphere.  The  curve  e  c  is 
therefore  adiabatic.  In  all  cases  let 

/  be  the  initial  temperature  of  the  air  in  absolute  degrees  Centi- 

grade. 

T  the  absolute  temperature  after  explosion  or  heating. 
T  '  the  absolute  temperature  of  the  gases  after  adiabatic  ex- 

pansion. 

/  the  atmospheric  pressure. 
PO  the  absolute  pressure  of  the  explosion. 
v0  the  volume  at  atmospheric  temperature  and  pressure. 
v  the  volume  at  the  termination  of  adiabatic  expansion. 

In  the  particular  case  of  diagram  fig.  n,  where  the  expansion 
is  continued  to  the  atmospheric  line,  the  formula  expressing  the 
efficiency  is  very  simple.  Calling  K^,  the  specific  heat  of  air  at 
constant  volume,  and  K^the  sp.  heat  at  constant  pressure,  then 
the  heat  supplied  to  the  engine  is 

H  =  Kt.(T-  /), 


Thermodynamics  of  the  Gas  Engine 


43 


£    i:§ 

§=*?      * 

"p 

•2  S 

-  :?  -?        rt 

tfl 


8. 
>-, 

H 


0  o  o  o  o  o 


44  The  Gas  Engine 

and  the  heat  discharged  from  it  is 

HI  =  K,(T'  -/) 
therefore  efficiency  is 

K,--1 


MX-/) 
and  -±=y 

K,   . 

(T1   —  t\ 
I.  (l) 

T    -   // 

It  is  evident  that  for  every  value  of  T  there  is  a  corresponding 
value  of  x1,  which  increases  with  the  increase  of  T.  If  xl  is 
known  in  teims  of  T,  then  the  calculation  of  efficiency  is  very 
rapid,  as  all  that  is  required  is  a  knowledge  of  the  maximum 
temperature  of  the  explosion  to  calculate  the  efficiency  of  an 
engine  using  that  maximum  temperature,  and  perfectly  fulfilling 
this  cycle. 

For  any  adiabatic  curve,  the  pressure  multiplied  by  volume 
which  has  been  raised  to  the  _>th  power  is  a  constant  ;  there- 
fore 

Pa  ?'/  =  A7'^  (see  diagram,  fig.  1  1),  (a) 

T         P  P 

and  —  =  _f  which,  as/  =A>  is  ^e  same  as    "  ; 

f     /  A 

also  11  =  II. 

v.      * 

.'.  in  equ  ition  (a)  x  may  be  substituted  for  P,,,  /  for  /„,  /  for  r0,  and 

r1  for  r,  giving 


T1   = 


In  most  engines  of  this  type  the  expansion  is  not  great  enough 
to  reduce  the  pressure  to  atmosphere  before  opening  the  exhaust 
valve  ;  it  is  therefore  necessary  to  give  formulae  where  the  best 
condition  is  not  carried  out.  Fig.  12  is  a  diagram  of  a  case  of 
this  kind. 

The  pressure  at  the  termination  of  the  stroke  has  fallen  to  A> 


Thermodynamics  of  the  Gas  Engine  45 

and  the  temperature  to  T1.     The  heat  supplied  to  the  engine  is 
the  same  as  in  the  first  case 

H  =  K..  (  T—  /). 

The  heat  discharged  by  it  cannot  be  so  simply  expressed. 
Suppose  the  hot  gases  at  the  pressure  /„  to  be  allowed  to  cool  by 
contact  with  the  sides  of  the  cylinder  at  constant  volume  till  the 
atmospheric  pressure/  is  reached,  then  the  temperature 

/»  =  T»   ^ 

A' 

or  in  terms  of  volume  and  /  tl  =  ~  f, 

''« 

and  the  heat  lost  is  KV  (x1  —  tl  ). 

The  heat  to  be  still  abstracted  before  the  air  returns  to  its 
original  condition  at  /,  and  pressure/  is 


Total  heat  discharged  by  exhaust,  therefore, 

H1  =  KZ,(T1  _/l)  +  K.(/»-/). 

The  efficiency  consequently  is 

K,  (T  -  /)  -  (K,  (T'  -  /')  +  K,  (/*  -  /)} 
MX-/) 


X  -  t 

In  this  case  there  is  no  fixed  relationship  between  T  the  tempera- 
ture of  the  explosion,  and  x1  the  temperature  of  the  gases  at  the 
termination  of  adiabatic  expansion.  As  the  expansion  is  more  or 
less  complete,  so  does  x  and  x1  change.  In  no  case,  however, 
can  the  efficiency  be  so  great  as  that  in  the  first  case. 

Type  2.  —  A  perfect  indicated  diagram  of  an  engine  of  this  type 
is  shown  at  fig.  13.  Although  the  cycle  requires  two  cylinders, 
producing  two  diagrams,  they  are  better  compared  when  super- 
posed. The  whole  diagram  may  be  supposed  to  come  from  the 
motor  cylinder,  the  shaded  portion  of  it  representing  the  available 
work  of  the  cycle,  and  the  unshaded  part,  the  part  done  by  the 
compressing  pump.  The  atmospheric  line  is  a  be.  The  pump 
volume  is  ab,  the  motor  volume  is  a  c.  The  pump  takes  in  the 
volume  a  b  at  atmospheric  pressure  ;  it  compresses  it  into  an 


46  The  Gas  Engine 

intermediate  receiver,  the  compression  line  (adiabatic)  is  bf, 
passing  into  receiver,  line  fe.  From  the  receiver  it  enters  the 
cylinder  at  the  constant  pressure  of  compression  on  the  line  efg, 
supply  of  heat  cut  off  at  g.  Then  expansion  (adiabatic)  to  the 
point  c  atmospheric  pressure.  The  part  bfgc  is  the  part  avail- 
able for  work,  the  part  bfea  representing  the  work  of  the  com- 
pressing pump,  which  is  deducted  from  the  total  motor  cylinder, 
diagram  aegc. 

The  total  volume  of  air  passed  through  the  pump  is  ?'„,  volume 
swept  by  motor  cylinder,  v.  So  far  as  the  heat  operations  are 
concerned,  the  part  of  the  diagram  to  volume  vc  may  be  disre- 
garded ;  it  represents  the  pressing  of  the  compressed  charge  into 
the  reservoir  after  reaching  the  maximum  pressure  of  compression 
(it  is  called  vc  because  it  is  volume  of  compression).  The  admis- 
sion to  the  motor  cylinder  is  identical,  so  that  work  done  in  pump 
in  that  part  equals  work  done  upon  the  motor  piston. 
In  addition  to  the  letters  used  in  type  i, 

.vc  is  volume  of  compression. 

vp  volume  at  point  g  on  diagram. 

pc  is  pressure  of  compression. 

/,.  is  temperature  of  compression. 

The  temperature,  volume,  and  pressure  letters  are  figured 
below  the  diagram  to  make  matters  clear.  Compression  is  carried 
on  from  volume  v0  at  atmospheric  pressure  and  temperature  to 
volume  vc  at  pressure  /,.  and  temperature  ta  the  curve  being 
adiabatic. 

After  compression,  heat  is  added  without  allowing  the  pres- 
sure to  increase,  but  the  piston  moves  out  till  the  maximum  tem- 
perature T  is  attained,  and  the  supply  of  heat  being  completely  cut 
off,  adiabatic  expansion  follows  till  the  atmospheric  pressure  is 
reached  ;  the  exhaust  valve  is  then  opened,  and  the  hot  gases  dis- 
charged. 

It  is  evident  that  as  the  pressure  is  constant,  while  heat  is 
being  given,  the  amount  of  heat  given  to  the  engine  in  all  is 

H  =  K,  (T  -  /,), 
and  the  heat  discharged  from  it  is  al.-o  at  constant  pressure, 


Thermodynamics  of  the  Gas  Engine 


47 


48  The  Gas  Engine 

The  efficiency  is  therefore 


-0 


•pi  / 

='-x-t  <4> 

The  compression  and  expansion  curves  being  adiabatic, 
Compression  /,.  ry  =  p  v/, 
Expansion  pc  v/  =  p0vy; 


so  that       =  .(a) 


vp      T  v      T1 

Substituting  in  equation  (a) 

T        T1' 

and  -  =  -  • 
T      /, 

As  the  efficiency  is 

x1  -  / 

T1  / 

it  may  be  either  =  i  -       or  =  i  —    -  (5) 

That  is,  when  expansion  is  carried  to  the  same  pressure  as  existed 
before  compression,  the  efficiency  depends  upon  the  compression 
alone,  /  being  the  temperature  before  compression,  and  tc  the  tem- 
perature of  compression.  The  efficiency  being  i  —  -,  the  greater 

the  temperature  /,.  the  less  is  the  fraction  - ,  and  the  more  nearly 

does  E  approach  unity. 

In  most  working  engines  of  this  kind,  the  expansion  is  not 
continued  long  enough  to  make  the  pressure  after  expanding  fall 
to  atmosphere  ;  so  that  the  efficiency  is  never  so  great,  as  when  that 
is  done,  a  greater  portion  of  the  heat  is  discharged  than  need  be. 


Thermodynamics  of  the  Gas  Engine  49 

The  modification  of  the  formulse  is  precisely  as  in  type  i  for 
similar  circumstances.  A  diagram  of  the  kind  is  shown  at  fig.  14. 
The  temperature  tl  is  found  as  before  : 

T   ~Po' 
The  heat  supplied  to  the  cycle  is  as  before  : 

H  =  K,  (T  -  /,), 
and  the  heat  discharged  is 

H1  =  K,  (x1  -  /')  +  K,  (/»  -  /). 
The  efficiency  is 

I  (Ti  _  /i)  +  (/i  _  /) 

^-'— T-* <6> 

Although  there  is  no  fixed  proportion  between  the  efficiency  and 
the  temperature  of  adiabatic  compression,  it  is  evident  that  E  in- 
creases with  increase  of  tc. 

Type  3. — A  perfect  indicator  diagram  of  an  engine  of  this  type 
is  shown  at  fig.  15.  As  in  type  2,  the  diagrams  of  pump  and 
motor  are  combined,  the  whole  diagram  being  that  given  in  the 
motor  cylinder,  but  the  shaded  portion  only  represents  the  avail- 
able work.  The  atmospheric  line  is  a  b  c.  The  pump  volume  is 
ab^  the  motor  cylinder  volume  is  a  c.  The  pump  takes  in  the 
volume  a  b  at  atmospheric  pressure,  compresses  it  on  the  adiabatic 
line  bf  and  into  a  receiver  on  the  I'mefg.  The  compressed  gases 
enter  the  motor  cylinder  on  the  line  gft  heat  is  added  instantane- 
ously, and  the  pressure  rises  on  the  line  fe.  Supply  of  heat  cut 
off  at  e  and  the  expansion  line  e  c  is  adiabatic.  The  total  diagram 
in  the  motor  cylinder  is  a g  f  e  c,  but  the  portion  agfb  is 
common  to  motor  and  pump  ;  the  available  work  is  therefore 
bfe  c. 

The  total  volume  of  air  passed  through  the  pump  is  vg ;  the 
volume  after  adiabatic  compression,  from  atmospheric  pressure  p 
and  temperature  /  to  pressure  of  compression  pc  and  temperature 
/„  is  V?  Heat  is  supplied  at  constant  volume  vc  till  the  maximum 
temperature  of  the  explosion  T  is  attained.  The  piston  then 
expands  the  hot  gases  adiabatically  from  temperature  T  to  T1 
and  pressure  p,  to  pressure  /„,  which  in  this  case  is  equal  to 
atmosphere. 


The  Gas  Engine 


The  heat  is  discharged  in  passing  from  volume  v  to  v0  at  con- 
stant pressure  of  atmosphere.  The  part  of  the  diagram  from 
volume  vc  to  zero  may  be  disregarded  as  it  is  common  to  both 
pump  and  motor. 

The  heat  supplied  to  the  cycle  is 

H  =  K7,  (T  —  tf). 


ooooooooooo 

o   y  oo  rx<o   m  f  F)  w  M 
S3.mSS3.Ij 


Thermodynamics  of  the  Gas  Engine  5  1 

Heat  discharged 

H'  =  K    (T1-/). 
The  efficiency  is 


E  = 


It  is  evident  that  for  any  maximum  temperature  T  and  com- 
pression temperature  tc  there  is  a  temperature  x1  at  which  the 
expansion  adiabatic  line  falls  to  atmosphere.  It  will  much  sim- 
plify subsequent  calculations  to  establish  the  relations  between  T, 
/t,  /  and  T1. 

PJZ'/  =  p0vv  and  pcv?  =  pv?  and  as  /„=/, 
p?_  v>_ 

PC  ~  V? 

bu,  I  =*'  so,  ha,     <  =  ^ 


,  «,     .    T  /T'V 

and  —  =  —  so  that  -  =  I  —  ) 

^       \t  I 


T          «,     .    T 

= 
PC 

x1  in  terms  of  T,  /,.  and  /  is  therefore 


Although  this  is  the  best  case  for  the  third  type  it  is  not  the 
one  commonly  occurring  in  practice  ;  no  engine  has  as  yet  been 
arranged  to  expand  the  gases  after  explosion  to  the  atmospheric 
pressure. 

Fig.  1  6  is  a  perfect  diagram  of  the  most  common  case,  namely, 
when  the  expansion  is  carried  only  so  far  that  the  heat  is  dis- 
charged when  the  volume  is  the  same  as  that  existing  before  com- 
pression. The  formula  of  efficiency  is  exceedingly  simple,  and 
leads  to  a  very  apparent  and  nevertheless  somewhat  paradoxical 
result. 

The  heat  supplied  to  the  cycle  is 

H  =  KV  (T  -  /,), 
and  the  heat  discharged  is 

H1  =  KV  (T>  -  /), 

E2 


52  The  Gas  Engine 

because  the  volume  of  the  air  is  the  same  as  that  existing  before 
compression,  and  therefore  the  heat  necessary  to  bring  the  fluid 
back  to  its  original  state  can  be  abstracted  at  constant  volume. 


300  r; 


•i      '2     '3     '4      '5     '6      "7      "8      -9 


..6 


VOLUMES. 

t  absolute  temp.  CC.  at  b       p  absolute  pressure  at  b 
U          „  „  /      £          ,,  ,,  f 

T  )f  „  e       Vo          volume  at          b 

T'          „  ,,  f        * 

vc  „ 

Here  Vo  =  v. 

FIG.  16. 


f 


Type  3.     Perfect  diagram.     Expansion  to  same  vol.  as  before  compression. 


The  efficiency  is 

_  K,  (T  -  /,)  -  K.  (Tl  ~  /) 
K.  (T  -  O 

Tl"/ 

T    -tt 


(9) 


As  both  curves  are  adiabatic,  and  pass  through  the  same  volume 
cnange, 


so  that 


Thermodynamics  of  the  Gas  Engine  53 

T1  -  /          T1         / 


T    —  tc         T          tc 

The  efficiency  may  therefore  be  expressed 

T1  / 

E  =  i  —    -     or  i (10) 

T  tc 

AvV" 

or          i  -     -         • 
\vj 

That  is,  the  efficiency  depends  upon  the  ratio  between  the 
initial  temperature  and  the  temperature  of  adiabatic  compression 
only.  T,  the  temperature  of  explosion,  may  be  any  value  greater 
than  tc,  without  either  increasing  or  diminishing  the  efficiency. 
In  this  case 

T'  =  !' 

There  is  still  another  case  of  this  type  of  cycle  to  be  con- 
sidered,  when  the  expansion  is  continued  beyond  the  original 
volume  before  compression,  but  not  carried  far  enough  to  reach 
atmospheric  pressure.  Fig.  17  is  a  diagram  of  the  kind. 

The  heat  supplied  to  the  cycle  is  still 
H  =  KJ,  (T  —  /f). 

The  heat  discharged  may  be  found  as  in  a  similar  case  with 
types  i  and  2. 

Total  heat  discharged  is 

H1  =  K,,  (T1  —  /')  f  K,  (/!  —  /). 

The  efficiency  is 


= 


Here  then  is  no  constant  relationship  between  x1  and  T  ; 
the  value  of  the  cycle  lies  between  cases  ist  and  2nd.  The 
efficiency  is  less  than  in  the  first  case,  but  greater  than  in  the 
second. 

Type  i  A.  —  In  this  type  of  engine  the  efficiency  cannot  be 
stated  in  terms  of  temperature  directly  because  of  the  nature  of  the 
perfect  cycle. 


54 


The  Gas  Engine 


'2     '3     '4     '5      '6     '7      -8      '9  'I     '2      '3      4     '5      '6     '7      '8      '9 


VOLUMES. 

/  absolute  temp.  CC.  at  b       p  absolute  pressure  at  b 

#        „          ..         /     A  „          ..         / 

P°  „  „  e 

T  „  „  e        TO  volume  at          b 

T'  c        v  „  c 

vc  ,.  / 

FIG.  17. — Type  3.     Perfect  diagram.  Incomplete  expansion. 


FlG.  18 — Type  i\.     Perfect  diagram.     Limited  expansion. 


TJiermodynamics  of  the  Gas  Engine  55 

The  expansion  line  is  adiabatic,  and  the  compression  line 
whereby  all  the  heat  is  discharged  is  isothermal.1 

Fig.  1 8  is  the  theoretical  diagram  of  such  an  engine.  The 
scale  is  altered  from  previous  diagrams  because  of  the  great  ex- 
pansion. 

There  is  no  compression  previous  to  the  addition  of  heat,  the 
heat  is  added  at  constant  volume  ?'„,  which  is  the  volume  of  the 
charge.  The  pressure  rises  with  the  temperature  from  atmos- 
pheric pressure  /  and  temperature  /  to  maximum  pressure  Pa  and 
temperature  T.  From  T  the  expansion  line  is  adiabatic,  and  is 
continued  far  enough  to  reduce  the  temperature  again  to  /.  The 
piston  then  returns,  compressing  the  gases  at  the  temperature  /  till 
the  original  volume  va  and  pressure  /  are  attained. 

For  any  two  temperatures  /  and  T  there  is  evidently  a 
constant  relationship  between  the  available  work  and  work  dis- 
charged as  heat.  As  in  expanding  from  highest  to  lowest  tem- 
perature the  temperature  falls  from  T  to  /,  the  whole  area  cf 
the  diagram  T  v0  v  /,  may  be  taken  as  the  heat  supplied  to  the 
cycle. 

The  heat  rejected  is  discharged  at  constant  temperature  /,  and 
is  equivalent  to  the  area  v0v  tt, 

For  any  adiabatic  curve  the  area  Tv0  v  t  is 

area  =  — - —  (P,  v,  —  p0  r).  (i2)r 

y  —  I 

For  any  isothermal 

7' 

area  ?'<,#//  =  pv0  Log.  e  —  (13) 

v0 
The  efficiency  is  therefore — 


(p,  v,  —  p0v} 


y  - 
(y-  i )(/*', Log. eii 


1  In  Dr.  A.  Witz's  able  work.  -Etudes  sur  les  moteurs  a  gas  tonnant,  he  falls 
into  the  error  of  supposing  both  expanding  and  compression  lines  of  this  type  adi- 
abatic,  and  he  accordingly  grea'ly  ovtr-estimates  the  efficiency  proper  to  it. 


56  The  Gas  Engine 

but,  as  the  line  of  compression  discharging  heat  is  an  isothermal, 
that  is,  the  temperature  is  kept  constant  at  /  during  compression 
from  the  lowest  pressure  to  atmosphere, 

/?'„  =  p0v  (Boyle's  law). 
The  efficiency  may  therefore  be  written 

(y- 

E  =  I 

(y-  0/Log. 


then  ^  =  5. 

/      /      \v 

The  efficiency  can  therefore  be  given  entirely  in  terms  of  T  and 
(y- 


In  the  case  where  the  expansion  is  not  carried  far  enough  to 
bring  the  temperature  of  explosion  down  to  the  temperature  of  the 
atmosphere,  the  efficiency  can  be  found  by  using  the  formulae 
12  and  13  to  get  proportions  of  available  and  total  work,  and  thtn 
get  from  the  nature  of  the  compression  curve  the  total  heat  dis- 
charged. As  this  is  variable  it  will  be  better  to  study  it  from  a 
numerical  example  later  on. 

The  diagram  given  is  the  best  possible  for  this  kind  of  cycle. 

EFFICIENCY  FORMULAE  FOR  THE  DIFFERENT  TYPES. 
The  general  formulae  for  efficiency  of  the  four  kinds  of  cycle 
are  as  follows. 

TYPE  i,  isf  Case  : 

T1  —  t 
E  =  i  —  y  .  (16) 

T'  in  terms  of  T  and  /: 


Thermodynamics  of  the  Gas  Engine  57 

2nd  Case  : 

E  =  i  -  <T ) +  y  l  - 

TYPE  2,  i 


T  —  / 

('7) 
(18) 

r-',' 

also  E  =  i  —     — , 
fc 


«  (Tl    -   '')   +   C1    -   0 

E  =  I    —   < ,  ,      . 

T  -  /,  (19) 

TYPE  3,  i.r/  GZJ<?  : 

E  =  I  ~  y  J 


T  — 4  (20) 

x1  in  terms  of  T  and  / : 

IT 
T1  =  /  ( I 


T-//  (21) 


also  E  =  i  —  — 
CVw : 

E  =  I    —   '— 


T  —  tc  (22) 

TYPE  i  A : 

t(y  -  i)  Log.  e  f|)^r« 

E=I ^rr~  (23) 

Those  formulae  will  be  found  very  convenient  in  rapidly  calcu- 
lating the  theoretical  efficiency  for  any  kind  of  diagram,  but  they 
do  not  throw  much  light  upon  the  relative  advantage  of  the  differ- 
ent types.  In  type  i,  for  instance,  it  is  apparent  that  efficiency 

' 

increases  with  increase  of  temperature  because  the  fraction 


58  The  Gas  Engine 

becomes  less  with  increase  of  T,  but  it  does  not  rapidly  become 
less  because  x1  also  increases  with  increase  of  x. 

In  type  2,  ist  case,  the  efficiency  is  quite  independent  of  T, 
and  is  dependent  only  on  the  ratio  between  /  and  /c  or  ?•„  and  rc. 
Increase  of  x  (maximum  temperature)  increases  the  available 
portion  of  the  engine  diagram,  and  therefore  the  average  pressure, 
but  without  altering  the  efficiency. 

TYPE  3. — With  this  type  it  is  easy  to  see  (ist  case)  that  the 
efficiency  is  greater  than  in  type  i,  but  only  a  numerical  example 
will  show  the  proportion. 

In  the  second  case  it  may  be  greater  or  less  than  in  type  i, 
depending  altogether  on  the  amount  of  the  compression. 

To  obtain  a  clear  idea  of  the  relative  values  of  the  efficiencies, 
it  is  necessary  to  calculate  a  few  numerical  examples. 

CALCULAXED  EXAMPLES  OF  EFFICIENCY  OF  XHE  TYPES. 

Numerical  Examples. — Using  air  as  the  working  fluid,  the 
value  of }',  the  ratio  of  specific  heat  at  constant  volume  to  specific 
heat  at  constant  pressure  is  i  '408. 

-^  =  y  =  i  -408. 

*• 

The  gaseous  mixture  used  in  a  gas  engine  differs  considerably 
from  pure  air  in  its  composition,  and  consequently  in  the  ratio 
between  specific  heat  at  constant  volume,  and  specific  heat  at  con- 
stant pressure,  but  it  is  advisable  in  the  first  place  to  consider  the 
cycle  as  using  air  pure  and  simple.  So  many  circumstances 
modify  the  theoretic  efficiency  in  actual  practice  that  they  can  be 
best  considered  after  studying  the  simpler  cases. 

The  temperature  1600°  C.  is  a  very  usual  one  in  the  cylinder 
of  a  gas  engine,  and  it  will  be  calculated  in  each  instance  as  the 
maximum,  17°  C.  being  taken  as  atmospheric  temperature. 

A  similar  set  with  1000°  C.  as  the  maximum  will  be  calcu- 
lated to  show  in  each  case  the  change  of  efficiency,  if  any,  with 
change  of  maximum  temperature. 

TYPE  i. — ist  Case.  The  expansion  is  continued  to  atmos- 
pheric pressure. 

Taking    x  =  1600°  C.  =  1873°  absolute. 
/  =      17°  C.  =    290°      „ 


^Thermodynamics  of  the  Gas  Engine  59 

Then      x1  =  the   temperature   after  adiabatic   expansion   to 
atmospheric  pressure. 


T  -'Ur 

x1  =  290  ( L_I3)iiio8  _  I0g0o  absolute. 

V  290  /  , 

The  efficiency  is 

x1  —  /  0  1090  —  290 

E  =  i  —  y  -     -  =  i  —  1-408  -          --?—  =  0-29 
T  —  /  1873  —  290 

E  =  0-29  with  maximum  temperature  of  1600°  C. 
Taking  the  maximum  temperature  of  explosion  as  1000°  C. 

Absolute 
T  =     1273°  =  1000°  C. 

/=    290°=      17°  C. 
then    T1  =     829°. 

E  =  I   —   1*408         " 9 —  =  O'2t. 

1273  -  290 

E  =  0*23  with  maximum  temperature  of  explosion  as  1000°  C. 

In  this  cycle  the  efficiency  evidently  increases  with  increase  of 
the  temperature  of  the  explosion,  but  not  in  proportion  to  the 
increase  of  temperature  ;  a  change  of  maximum  temperature  from 
1000°  to  1600°  C.  only  causing  the  efficiency  to  rise  from  0-23 
to  0-29.  That  is,  at  the  first  temperature,  23  heat  units  out  of 
every  100  given  to  the  cycle  will  be  converted  into  work, 
while  with  the  second  much  higher  temperature,  only  29  units  of 
100  will  be  converted  into  work. 

The  second  case  of  this  type  is  the  one  most  commonly  occur- 
ring in  practice.  The  cylinder  is  so  arranged  that  the  charge  is 
taken  in  for  half- stroke,  the  explosion  then  occurs,  and  the  piston 
completes  its  stroke,  expanding  the  heated  gases  from  one  volume 
to  two  volumes. 

In  the  diagram,  fig.  12,  suppose  volume  v  to  be  equal  to 
2  ?'„,  and 

T  =  1873°  absolute. 
/=     290° 

To  get  T1, 


60  The  Gas  Engine 

/VV"1 

T1  =  T  (  —  I 
\VJ 

/  J  \  0-408 

T1  =  1873  f  —  j       =  1411°  absolute. 

To  calculate  efficiency  /]  is  still  required  ;  it  is,  in  terms  of 
Volume  and  /, 

/i  =  I'/  =  ^290  =  580°  absolute. 
The  efficiency  can  now  be  obtained  from  formula  (17). 

E  =   I   -  (T1   -/')+>>  (/'-/) 
T  —  / 

_  i        (141*  -  580)  +  1-408(580  -  290) 
1873  -  290 

=  x  _  «3L±_?.:4o8_X_2_9o  =  0.22  nearl 

1583 

For  this  case  E  =  o-22, 

showing  the  effect  of  limiting  the  expansion  and  discharging  at  a 
pressure  above  atmosphere. 

Taking  the  same  ratio  of  expansion  and  the  lower  maximum 
temperature  of  1000°  C. 

T  =  1273°  absolute. 
/  =    290° 

/7/>\  y-t  /  j  \  o  408 

as  before,  x1  =  T(  -  )       =  1273  [  — )       =  959°  absolute, 

\V-J  \2/ 

and  /'  is  still  290  x  2  =  580°  absolute. 
Therefore  E  —  o-2o. 

Here  the  diminution  of  efficiency  due  to  diminished  expan- 
sion is  not  so  great  as  in  the  first,  or  rather  the  higher,  tempera- 
ture, 

with  complete  expansion     1000°  C.  giving  0-23, 
„    limited  „  1000°  C.        „     0-20  ; 

with  the  higher  temperature  of  1600°  C., 

with  complete  expansion     1600°  C.  giving  0-29, 
„     limited  „  1600°  C.      „      o-22. 

It  is  evident  from  these  results  that  where  the  amount  of  ex- 


Thermodynamics  of  tJie  Gas  Engine  61 

pansion  is  from  one  volume  to  two  volumes,  as  in  the  Lenoir  and 
Hugon  engines,  the  efficiency  does  not  substantially  improve  with 
increasing  temperature. 

TYPE  2.  —  \st  Case.  Where  the  expansion  is  carried  far  enough 
to  reduce  the  working  pressure  to  atmosphere,  the  efficiency  of 
this  kind  of  engine  is  quite  independent  of  the  temperature  of 
combustion.  This  is  shown  by  Professor  Rankine  !  in  his  work  on 
the  steam  engine.  Whether  the  heat  added  after  compression 
be  great  or  small  in  amount,  the  proportion  of  it  which  is  con- 
verted into  work  is  stationary. 

The  compression  most  commonly  used  in  this  kind  of  engine 
is  60  Ibs.  per  sq.  in.  above  atmosphere,  75  Ibs.  per  sq.  in.  absolute, 
taking  the  atmospheric  pressure  as  15  Ibs.  per  sq.  in. 

The  compression  is,  as  before  stated,  adiabatic  ;  no  heat  is 
lost  or  gained.  The  temperature  rises  simply  because  of  work 
performed  upon  the  air. 

Let 

Atmospheric  temperature  and  pressure  (absolute)  /,/  =  290°  —  15 
Compression,  „  „  „  tapc—  -75 


(7  nX  °  -29 
Jj\      =  462-5°  absolute, 


200 

E  =  I   —   —  -  —  =  0'37 

462-5 

E  =  0-37. 

This  result  is  much  better  than  any  obtained  with  the  first 
type.  It  holds  equally  good  for  all  combustion  temperatures  ; 
with  either  1000°  C.  or  1600°  C.  the  efficiency  would  still  be 
0-37,  so  long  as  that  degree  of  compression  was  used.  With  a 
higher  compression  the  efficiency  increases  ;  100  Ibs.  per  sq.  in. 
above  atmosphere  is  quite  a  workable  degree  of  compression.  It 
is  instructive  to  calculate  the  efficiency  with  this  pressure  : 

1   The  Steam  Engine,  Prof.  Rankine,  p.  373,  Formula  (7). 


62  The  Gas  Engine 

t   =  290°  absolute. 

/  =    15  Ibs.  per  sq.  in.  absolute. 

PC  =  1  15        »      »       »       ?> 

/jjcXo-ag 

/,  =  290  I  -  -  1      =  524°  nearly. 


524 

E  =  0-45. 

This  type  is  evidently  much  superior  to  the  first  type,  as  it  i? 
capable  of  greatly  increased  efficiency  by  the  mere  increase  of 
compression. 

In  the  engines  in  practice  expansion  has  not  been  carried  far 
enough  to  give  the  results  calculated  above.  It  has  been  usual  to 
construct  the  engine  so  that  the  compression  pump  is  one-half  of 
the  volume  of  the  motor  cylinder,  that  is,  the  ratio  of  the  expan 
sion  is  from  one  volume  to  two  volumes  at  atmosphere.  Taking 
first  a  compression  of  60  Ibs.  per  sq.  in.  above  atmosphere  with  this 
proportion  between  the  volumes  at  atmosphere,  and  the  highest 
temperature  as  1600°  C.,  then  (diagram,  fig.  13) 

T  =  1873°  absolute 

/  =  290°        „ 

^=462-5°     . 

/l=  290    X    2  =  580. 

Before  getting  T1  it  is  necessary  to  get  the  volume  rf  at  the 
highest  temperature.  It  is 


and          vc  —  v0  f—  J  '  =  i  fS  V*  =  0-318 
.M>  =  0-318  Igi  =  1-29 


and         T>  =  x^        =  1873    i^?=  1566°  absolute. 
The  efficiency  can  now  be  found  by  formula  (19) 


E 


=  I  —  y~  -  =1  --L 


T  —  /,  1873  —  462-5 


Thermodynamics  of  the  Gas  Engine  63 

071  (986)  4-  200 

=  i  --  '-  —  ^  —  '—  '  —  Z-   =  i  —  070  =  o.  10 
1410-5 

E  =  0-30. 

Here  the  insufficient  expansion  has  caused  the  efficiency  possible 
from  the  compression  to  fall  from  0*37  to  0*30. 

Calculating  in  the  same  way  for  the  greater  compression  of 
100  Ibs.  per  sq.  in.  above  atmosphere,  with  expansion  ratio  between 
compression  and  motor  cylinders  of  two,  it  is  found  that  the  result 
is  improved. 

Here  vc  =  0-235  v°l- 

and  vf  =  0-841  vol. 


T'  =  T  =  1873    -  =  1318°  absolute. 

\V  I  \       2      / 


t  =  290° 

/l=  580° 

*c=  524° 

The  efficiency  is  therefore 


£  =  I  —  -L =  j  _— '- 

T-ff  1873-524 

_  0-71  x  738  +  290   _   _    814  _ 
1349  *349 

E  =  0*40. 

The  greater  compression  has  greatly  increased  the  efficiency 
while  leaving  the  proportion  of  the  two  cylinders  unaltered. 

Still  using  the  same  cylinders,  the  efficiency  with  compression 
of  60  Ibs.  above  atmosphere  and  a  maximum  temperature  of 
1000°  C.,  is 

E  =  0*28  nearly, 
the  data  being 

T'=     892°  T  =  1273° 

/,  =    462°, 
volumes 

Va=.  I  V     =         2 

{7,=  0-3I8  Vf-=      0-87. 


§4  The  Gas  Engine 

Using  the  higher  compression  100  Ibs.  above   atmosphere  with 
1000°  C.  as  highest  temperature 

E  =  0*44. 

Data  :  T'=     763°  T  =  1273° 

/!  =     580°  /   =    290° 

f<  =    524° 
Vol.  :  v,  =         i  v  =        2 

vc=        °'235  *>  =       °'57 

In  this  kind  of  engine  the  best  result  is  always  obtained  when 
the  expansion  is  carried  to  atmospheric  pressure.  The  necessary 
proportion  between  the  two  cylinders,  to  accomplish  this,  depends 
on  two  things  :  the  temperature  of  compression,  and  the  tempera- 
ture of  combustion.  The  ratio  between  the  cylinders  should  be 


With  a  temperature   of  compression   of  462°,   for   instance, 
and  a  maximum  of  1873°  absolute  \~*  =  4  '05  )  the  volume  of 

\402  / 

the  motor  cylinder  would  require  to  be  4-05  times  that  of  the 
pump.      With   the  increased   compression  giving  524°  absolute 

(  —  13-=  3*57)  ratio  of  motor  to  pump  3-57  to  i. 
\524  / 

With  the  lower  maximum  temperature  of  1273°  the  ratios  for 
the  two  compression  values  are 


2.ye  L273  _  2-43  nearly. 

462  524 

These  figures  explain  why  the  efficiency  varies  so  much  with 
two  cylinders  of  ratio  i  to  2  with  change  of  maximum  temperature 
and  compression. 

TYPE  3.  —  ist  Case.  In  this  case  expansion  is  carried  to  atmo- 
sphere. It  is  evident  from  the  formulae  that  efficiency  varies  to 
some  extent  with  maximum  temperature  of  the  explosion. 

Taking  first  a  maximum  temperature  of  1600°  C.,  as  in  the  last 
type  calculated,  with  a  pressure  of  compression  60  Ibs.  above 
atmosphere, 

The  data  are  as  follows  : 

Temperatures  T  =  1873°         /  =  290 

/,  =  462. 


Thermodynamics  of  the  Gas  Engine  65 

T'  in  terms  of  x  and  /,  /,.  is  (see  p.  57) 


T'   =   783°- 

The  efficiency  therefore 

E  =  i  -  y  Tl~'   =  i  -  1-408 


"  29° 


-  tc  1873  -  462 

E  =  0-51. 

With  compression  100  Ibs.  above  atmosphere, 
tt  =  524° 

and  x1  is  therefore  x1  =  200   f£5L?ir«Bi  =  c^e0 

\524j 

and  E  =  i  —  i '400 — — ~ — 

1873  —  524 

E  =  073. 

Taking,  next,  .1000°  C.  as  the  highest  temperature,  first  with 
the  lower  compression,  and  after  with  the  higher  compression, 
with  60  Ibs.  compression  x1  is  595°  absolute 
with  100  „  x1  is  545°         „ 

E  =  0-47  at  60  Ibs.,  E  =  0-52  at  100  Ibs.,  with  1000°  C. 

In  this  case  the  efficiency  varies  both  with  the  maximum  tem- 
perature of  the  explosion  and  the  compression  temperature  previous 
to  explosion.  A  glance  at  the  numbers  placed  together  will  show 
clearly  the  relationship. 

Max.  temps,  in  CC.           ...  1600°           1600°  1000'-  looo3 
Pressure  of  compression  above  atmo- 
sphere         .....  60  Ibs.  icolbs.  60  Ibs.  loolbs. 
Efficiency        .....       o'5i            073  0-47  0^52 

2nd  Case. — Here  the  expansion  after  explosion  is  not  carried 
on  far  enough  to  reduce  the  pressure  to  atmosphere.  It  terminates 
when  the  volume  is  the  same  as  existed  before  compression,  that  is, 
the  volume  swept  by  the  motor  piston  in  expanding  doing  work  is 
identical  with  that  swept  by  the  pump  piston  in  compressing  up  to 
maximum  pressure.  Pump  and  motor  are  equal  in  volume.  To  this 
case  of  type  3  belong  all  compression  engines  in  which  the  motor 
piston  compresses  its  charge  into  a  space  at  the  end  of  the  cylinder. 

F 


66  The  Gas  Engine 

In  this  case,  as  in  case  i,  type  2,  the  theoretic  efficiency  of  the 
engine  is  quite  independent  of  the  maximum  temperature  of  the 
explosion.  So  long  as  the  volume  after  expansion  is  the  same  as 
that  before  compression,  it  does  not  matter  in  the  least  how  much 
heat  is  added  at  constant  volume  of  compression ;  whether  only  a 
few  degrees  rise  occurs  or  1000°  or  2000°,  it  is  all  the  same  so 
far  as  the  proportion  of  added  heat  converted  into  work  is  con- 
cerned. That  proportion  depends  solely  upon  the  amount  of 
compression. 

For  60  Ibs.  adiabatic  compression,  temperature  462°  absolute, 
the  efficiency  is  0^37 ;  for  100  Ibs.  above  atmosphere  it  is  0-45.  Given 

by  the  formula  E  =  i  —    •-  .    (See  p.  57.) 

*t 

E  depends  absolutely  upon  the  temperature  of  the  atmosphere 
and  the  temperature  of  compression  t  and  /,,.  If  the  relative 
volumes  of  space  swept  by  piston  and  compression  space  be  known, 
then  the  efficiency  can  be  at  once  calculated. 

yd  Case. — Here  the  expansion  is  carried  further  than  the 
original  volume  before  compression,  but  not  far  enough  to  reduce 
the  pressure  to  atmosphere.  Efficiency  is  always  less  than  in  the 
first  case  with  corresponding  temperature  of  explosion  and  compres- 
sion, but  greater  than  in  the  second  case.  It  is  found  by  the 
formula  : 

(T'  -  /')  +  y  (/'  -  Q 

T    -    tc 

/*  depends  on  the  relationship  between  the  volumes  v0  and  v  the 
volume  at  atmosphere  and  the  volume  of  discharge  after  expansion, 
it  is  always  : 

/•  =  /  <-  - 
v, 

T1  is  also  found  by  the  same  method  as  in  types  i  and  2.  It  is 
better  to  postpone  calculating  any  particular  case  of  this  at  present, 
as  no  engine  doing  this  has  yet  got  into  public  use,  and  it  can  be 
considered  further  on  in  discussing  the  effect  of  increased  expan- 
sion in  the  actual  engines. 

Type  i  A. — The  efficiency  of  this  type  of  heat  cycle  depends 
to  a  considerable  extent  upon  cooling  during  the  return  stroke  ; 
in  its  best  form,  cooling  at  the  lowest  temperature  during  isother- 


Thermodynamics  of  the  Gas  Engine  67 

mal  compression,  it  cannot  be  carried  out  without  introducing  the 
very  disadvantages  with  which  the  hot-air  engine  was  saddled, 
namely,  a  dependence  upon  the  slow  convection  of  air  for  the 
discharge  of  the  heat  necessarily  rejected  from  the  cycle.  The 
rapid  performance  of  this  operation  is  impossible,  and  accordingly 
it  is  hardly  fair  to  compare  this  type  with  those  preceding  ;  they 
could  all  of  them  be  greatly  improved  in  theory  by  introducing 
greater  expansions  and  cooling  by  convection  at  the  lowest  tempe- 
rature, but  all  at  the  expense  of  rate  of  working.  The  efficiency 
of  type  i  A,  will  be  found  to  be  high  ;  but  it  is  to  be  kept  con- 
stantly in  mind  that  the  penalty  of  slow  rate  of  work  was  fully 
exacted  in  the  practical  examples  of  the  kind  in  public  use.  They 
are  exceedingly  cumbrous,  and  give  but  a  trifling  power  in  com- 
parison with  their  bulk  and  weight.  The  efficiency  in  this  type  is 
dependent  upon  T  and  /  only. 


E=  j  _ 


T 


1  -  f 

Take  first  T  =  1873° 

/=    290° 
E_  j  _  0-408  /  x  4-57 

E  =  0-66. 

This  is  a  very  high  efficiency,  but  it  is  obtained  by  using  an 
enormous  expansion, 

—  =  (  —  p-1  =  967  nearly. 

The  piston  must  move  through  nearly  100  times  the  original 
volume  of  the  charge  before  the  temperature  is  reduced  to  the 
temperature  existing  before  igniting  ;  in  passing  back  to  unit 
volume  the  gases  must  be  supposed  to  keep  at  /  by  the  cooling 
effect  of  the  cylinder  walls. 

When  T  =  1000°  C.  =  1273°  absolute, 

t=      17°  C.  =    290°       „ 
the  efficiency  is 

E  =  0-56, 
and  the  expansion  required  is  not  so  great,  being  37-5  volumes. 


63 


The  Gas  Engine 


The  actual  ratios  of  expansion  used  in  practice  have  not  ap- 
proached those  proportions,  and  will  be  considered  while  discuss- 
ing the  diagrams  taken  from  engines  of  this  type. 

COMPARISON  OF  RESULTS. 

The  two  maximum  temperatures  used,  1600°  C.  and  1000°  C., 
with  the  lowest  temperature,  17°  C.,  give  in  a  perfect  heat-engine, 
efficiencies 

1600°  C.  =  0-85  nearly, 

1000°  C.  =  077      „ 

One  case  in  type  3  comes  nearer  to  a  perfect  heat-engine  than 
any  of  the  others.  To  compare  easily  the  following  table  will  be 
useful. 

TABLE  OF  THEORETIC  EFFICIENCY. 


Max.  temp.  °C. 

Compression 

Efficiency 

Temp. 

Pressure 

Type  i. 

abs.°C. 

above  atmos. 

Expanding  to  atmosphere 

1600° 

— 

— 

O'zg 

»            ..            » 

1000° 

— 

— 

0*23 

Expanding  to  twice  volume 

\          loco3 

— 

— 

O'22 

existing  before  ignition 

r     1000° 

— 

— 

O'2O 

Type  2. 

Expanding  to  atmosphere 

— 

462° 

6olbs. 

0-37 

ii            it            ii 

— 

524° 

loo  Ibs. 

°'45 

/           l6qo° 

462° 

60  IDS. 

°'3O 

Expanding  to  twice  volume 

1600° 

524° 

100  Ibs. 

o'4O 

existing  before  compression 

1000° 

462° 

60  Ibs. 

0-28 

I            1000° 

524° 

100  Ibs. 

0-44 

Type  3. 

Expanding  to  atmosphere 

1600° 

462° 

60  Ibs. 

0-51 

.1            ii            •• 

1600° 

524° 

loo  Ibs. 

073 

it            i»            ii 

1000° 

462° 

60  Ibs. 

0-47 

ii            ii            ii 

I0005 

524° 

100  Ibs. 

0-52 

Expanding    to     the    same 

) 

volume  as  existed  before 

1 

462° 

60  Ibs. 

0*37 

compressing 

}                   ( 

524° 

ico  Ibs. 

°'45 

Expanding  to  greater  volume 
than  existed  before  com- 
pressing, but  not  enough 

'  Efficiency  between  ist  and  2nd  cases  of  this  type 
depending  on  ratio  of  expansion. 

to  reach  atmosphere 

) 

Type  i  A. 

Expanding  from  max.  temp. 

1600°                        — 

0-66 

to  lowest  temperature 

1000°                                   — 

0-56 

Thermodynamics  of  the  Gas  Engine  69 

Comparing  first  the  best  results  of  each  type,  it  is  evident  that 
type  i  is  the  least  perfect  as  a  heat-engine,  giving  back  only  0-29 
of  the  total  heat  entrusted  to  it  as  mechanical  work,  and  rejecting 
the  rest  of  the  heat.  Type  2  is  distinctly  better,  giving  a  maximum 
efficiency  of  0*45,  or  nearly  half  the  heat  converted  into  work. 

Type  i  A,  with  a  heat  conversion  of  0-66,  is  still  better  ;  but 
tyPe  3»  with  °'73  efficiency,  is  best  of  all,  coming  very  closely 
indeed  to  what  a  perfect  cycle  is  capable  of  giving. 

It  cannot  be  too  constantly  kept  in  mind  that  it  by  no  means 
follows  that  the  best  theoretic  efficiency  will  give  the  best  result  in 
practice.  If  gained  at  the  expense  of  great  volume  or  an  imprac- 
ticable process,  it  may  not  be  worth  so  much  as  a  worse  cycle 
where  small  volume  of  cylinder  and  an  easy  process  make  it  more 
easily  attainable.  Type  i  A  is  at  a  great  disadvantage  in  the 
matter  of  expansion  ;  it  requires,  as  has  been  shown,  expansion  of 
967  and  37-5  volumes  respectively,  so  great  that  it  is  practically 
out  of  comparison  as  a  workable  cycle  with  the  others.  The 
other  cycles  vary  in  this  respect  also,  but  the  variation  will  fall 
under  the  consideration  of  mechanical  efficiency  at  a  later  stage. 
Type  i  A  is  so  much  out  that  it  was  necessary  to  mention  it  here. 

In  type  i,  the  efficiency  varies  with  the  temperature  of  explo- 
sion, especially  where  the  expansion  is  carried  to  atmosphere  ;  the 
difference,  however,  is  not  great,  a  very  large  increase  of  maximum 
temperature  but  slightly  increasing  the  efficiency,  1000°  C.  giving 
0-23,  and  1600°  C.  only  0-29,  of  heat  conversion.  When  the 
expansion  is  limited  to  twice  the  volume  at  the  moment  of  heat- 
ing, the  effect  of  increasing  temperature  in  increasing  the  efficiency 
is  almost  nil,  1000°  C.  giving  o-2o  efficiency  s  and  1600°  C.  only 
0-22  efficiency.  The  conclusion  to  be  drawn  from  the  fact  is  this  : 
in  engines  of  the  Lenoir  or  Hugon  kind,  with  limited  expansion, 
the  economy  is  not  increased  by  using  high  temperatures  ;  a  weak 
mixture  will  give  as  good  an  indicated  efficiency  as  a  strong  one. 

With  type  2,  the  maximum  efficiency  is  obtained  by  expand- 
ing to  atmospheric  pressure,  and  in  this  case  it  is  quite  independent 
of  the  temperature  of  combustion;  it  does  not  matter  whether  a  great 
or  small  increase  of  temperature  occurs  at  the  pressure  of  compres- 
sion, the  efficiency  remains  the  same.  That  is,  whether  much  heat 
be  added  or  little  heat,  the  proportion  converted  into  work  depends 


/  o  Tlie  Gas  Engine 

on  one  thing  only,  that  is,  the  amount  of  compression  —  the 
greater  the  compression  the  greater  the  efficiency  of  the  engine. 
The  pressures  of  compression  which  have  been  calculated,  are 
pressures  which  have  been  used  for  the  kind  of  cycle  in  practice. 
The  only  limits  to  increasing  compression  are  the  practical  ones  of 
strength  of  engine  and  leakage  of  piston.  The  difference  between 
efficiency  at  60  Ibs.  and  100  Ibs.  compression  above  atmosphere  is 
considerable,  the  first  giving  E  =  0-37,  the  second  E  =  0-45. 

When  the  expansion  is  limited  to  twice  the  volume  existing 
before  compression  the  maximum  temperature  then  affects  the  effi- 
ciency, but  not  to  such  an  extent  as  the  compression. 

Type  3. — This  is  the  best  type  of  all  from  the  point  under  con- 
sideration. The  efficiency  in  the  best  form  of  it  varies  both  with 
maximum  temperature  and  pressure  of  compression.  At  1600°  C. 
maximum  temperature  and  compression  60  Ibs.  per  square  inch 
above  atmosphere,  E  =  0-51.  At  the  same  maximum  tempera- 
ture but  the  higher  compression  of  100  Ibs.  above  atmosphere, 
it  rises  to  the  high  efficiency  of  073,  which  is  very  nearly  what  a 
perfect  heat-engine  using  1600°  C.  and  atmospheric  temperature 
could  give.  With  maximum  temperature  of  1000°  C.  for  these 
two  compression  pressures  the  efficiencies  are  E  =  0*47  and  E  = 
0-52.  The  best  case  of  this  type  is  not  the  one  occurring  in 
practice,  in  fact  no  compression  engine  of  this  kind  has  ever  been 
much  which  expands  to  atmosphere  Usually  expansion  is  only 
carried  to  the  same  volume  as  existed  before  compression,  and 
there  the  efficiency  is  quite  independent  of  the  maximum  tempera- 
ture; it  is  determined  by  compression  solely  as  in  typo  2. 

For  compression  60  Ibs.  per  square  inch  above  atmosphere  it 
is  o-37,  and  for  100  Ibs.  per  square  inch  above  atmosphere  it  is 
0-45,  the  difference  between  types  2  and  3  in  this  case  being, 
that  type  2  expands  its  working  fluid  at  the  pressure  of  compres- 
sion, which  remains  constant,  and  the  pressure  falls  to  the  pressure 
of  atmosphere  by  the  movement  of  the  piston  doing  work;  in 
type  3  the  heat  is  added  to  the  working  fluid  at  constant  volume, 
pressure  increasing,  then  expansion  doing  work,  till  volume 
before  compression  is  attained.  The  one  acts  by  increase  of  the 
volume  of  the  working  fluid  by  heat,  the  other  by  increase  of 
pressure  of  the  working  fluid  by  heat.  The  one  engine  gives 


Thermodynamics  of  tJie  Gas  Engine  7 1 

large  volumes,  Icr.v  pressures;  the  other  small  volumes,  high 
pressures. 

In  type  i  A,  the  change  of  volume  required  is  so  great  that  its 
efficiency  cannot  be  fairly  compared  with  the  others. 

Conclusions. — The  best  cycle  for  great  efficiency  is  produced 
by  using  compression  in  the  manner  of  type  3 

In  any  cycle  with  any  definite  expansion,  increase  of  compression 
previous  to  heating  produces  increase  of  the  proportion  of  heat 
converted  into  work.  In  some  cases  of  compression  cycles,  increase 
of  the  highest  temperature  does  not  increase  the  efficiency; 
it  may  even  diminish  it. 

There  are  cases  in  types  2  and  3  when  the  efficiency  is  quite 
independent  of  the  maximum  temperature,  depending  solely  on 
the  amount  of  compression  employed 


72  The  Gas  Engine 


CHAPTER   IV. 

THE   CAUSES    OF    LOSS    IN    GAS    ENGINES. 

IN  calculating  the  efficiency  of  the  different  kinds  of  engines,  it 
has  been  assumed  that  the  conditions  peculiar  to  each  cycle 
have  been  perfectly  complied  with.  In  actual  engines  this  is 
impossible  ;  it  is  therefore  necessary  to  discover  in  what  manner 
practice  fails  in  performing  the  operations  required  by  theory. 

The  actual  engines  differ  from  the  ideal  ones  in  several 
ways  : 

1.  The  working  fluid  loses  heat  to  the  walls  enclosing  it  after 
its  temperature  has  been  raised  to  the  highest  point ; 

2.  The   working   fluid   often   gains  heat   when   entering  the 
cylinder  at  a  time  when  it  should  remain  at  the  lowest  tempera- 
ture; 

3.  The  supply  of  heat  is  never  added  instantaneously  as  is  re- 
quired in  some  types ; 

4.  The  working  fluid  does  not  behave  as  a  perfect  gas  ;  owing 
to  the   complex  phenomena  of  combustion,  to  some  extent  its 
physical  state  is  changed  during  the  addition  of  heat ; 

5.  The  admission,  transfer  and  expulsion  of  the  working  fluid 
are  not  accomplished  without  some  resistance,  wire-drawing  during 
admission,  back-pressure  during  exhaust. 

The  first  cause  of  loss  is  by  far  the  most  considerable  and  will 
be  considered  first. 

Loss  OF  HEAT  TO  THE  CYLINDER  AND  PISTON. 

Although  this  is  the  most  considerable  source  of  loss  in  all 
gas  engines,  the  stock  of  information  in  existence  upon  the  subject 
is  quite  insufficient  to  justify  any  attempt  to  state  a  general  law. 
So  far  as  the  author  is  aware,  no  experiments  have  yet  been  made 


The  Causes  of  Loss  in  Gas  Engines  73 

to  determine  the  rate  at  which  a  mass  of  heated  air,  at  from  1000° 
to  1600°  C.  loses  heat  to  the  comparatively  cool  metal  surfaces 
which  enclose  it.  That  the  rate  of  flow  is  rapid  is  quite  evident. 
Otherwise,  it  would  be  impossible  to  raise  steam  with  the  relatively 
small  heating  surfaces  generally  used  in  boilers.  Before  applying 
the  efficiency  values  obtained  to  actual  practice  it  is  necessary  to 
know  at  what  rate  a  cubic  foot  of  air  at  about  1600°  C.  in  contact 
with  metal  walls  at  from  17°  C.  to  100°  C.  will  lose  heat;  also  to 
know  how  that  rate  changes  with  change  of  temperature  and 
density.  Much  is  known  of  the  laws  of  cooling  at  lower  tem- 
peratures, but  little  positive  data  exist  for  temperatures  so  high 
as  those  occurring  in  the  gas  engine.  A  hot  gas  loses  heat  to  the 
colder  walls  enclosing  it  mainly  by  circulation  or  convection.  The 
conductivity  of  gases  for  heat  is  very  slight,  and  unless  in  some 
way  a  large  surface  of  the  gas  is  exposed  to  the  cooling  surface, 
practically  no  heat  would  escape  from  the  working  fluid  in  the  short 
time  during  which  it  is  exposed  in  gas  engines.  Any  arrangement 
which  favours  or  hastens  convection  will  therefore  increase  loss  by 
increasing  the  extent  of  hot  gaseous  surface  -exposed  to  the  walls. 
The  smaller  the  surface  to  which  a  given  volume  of  working  fluid 
is  exposed  the  less  heat  will  it  lose  in  a  given  time.  So  far  as 
loss  of  heat  is  concerned  then,  the  best  type  of  engine  is  that 
which  exposes  a  given  volume  of  working  fluid  to  the  smallest 
surface  in  performing  its  cycle.  Suppose  that  in  the  three  types 
the  pistons  move  at  the  same  velocity,  then  that  which  requires  to 
move  through  the  smallest  volume,  the  areas  of  the  pistons  being 
supposed  equal,  will  take  the  shortest  time  to  perform  its  cycle.  In 
the  first  engine  the  piston  moves  through  27  vols.,  with  the  hot  air 
filling  the  cylinder;  the  second,  through  37  vols.;  and  the  third, 
through  2 -4  vols.  (see  diagrams  n,  13  and  15).  As  the  volumes 
are  proportional  to  the  time  taken  to  perform  each  cycle  the 
third  type  has  the  best  of  it,  the  time  of  exposure  of  the  hot 
working  fluid  being  the  least  ;  the  second  type  is  worse  than 
the  first.  There  is  still  another  circumstance  in  addition  to 
surface  exposed  and  time  of  exposure,  that  is,  the  average 
temperature  of  the  hot  gas  which  is  exposed.  If  the  average 
temperature  is  lower  in  one  type  than  in  another  during  ex- 
posure to  a  given  surface  for  a  certain  time,  then  obviously 


74  The  Gas  Engine 

less  heat  will  be  lost  in  the  one  than  in  the  other.  Comparing 
the  average  temperatures  it  is  found,  that  in  the  first  the  tempera- 
ture ranges  from  1600°  C.  to  817°  C. ;  in  the  second  from  1600°  C. 
to  901°  C.  ;  and  in  the  third  from  1600°  C.  to  510°  C.  The 
third  will  therefore  show  a  lower  average  temperature  than  the 
others.  Three  conditions  are  requisite  in  the  engine  which  is  to 
lose  the  minimum  of  heat  from  its  working  fluid  : 

1.  In  performing  its  cycle  it  should  expose   a  given   volume  of 

its  working  fluid  to  the  least  possible  cooling  surface ; 

2.  It  should  expose  it  for  the  shortest  possible  time ; 

3.  The  average  temperature  during  the  time  of  exposure  should 

be  as  low  as  possible — 

which  conditions  are  best  fulfilled  by  the  third  type.  In  ad- 
dition to  its  advantage  in  theoretic  efficiency  it  possesses  the 
further  good  points  in  practice  of  proportionally  small  cooling 
surfaces,  short  time  of  exposure,  and  rapid  depression  of  tempera- 
ture due  to  work  done,  consequently  small  loss  of  heat  to  the 
cylinder  and  piston. 

The  diagrams,  figs,  n,  13  and  15,  have  been  selected  from  the 
others  belonging  to  each  type  because  the  pressures,  temperatures, 
and  relative  volumes  closely  correspond  with  those  which  would 
be  best  and  at  the  same  time  readily  practicable. 

The  flow  of  heat  really  occurring  in  the  gas  engine  cylinder 
will  be  discussed  when  the  actual  diagrams  come  under  considera- 
tion ;  meantime,  it  is  sufficient  to  have  proved  that  the  third  type 
will  in  practice  give  results  more  closely  approaching  its  theory 
than  the  others.  If  in  each  case  a  constant  proportion  of  the 
heat  supplied  were  lost  to  the  cylinder  and  piston,  the  ratio  of  the 
efficiencies  would  remain  constant,  and  although  it  would  be  im- 
possible from  present  data  to  predict  the  actual  values,  yet 
the  relative  values  would  be  known. 

GAIN  OF  HEAT  BY  THE  WORKING  FLUID  WHEN  ENTERING 
THE  ENGINE. 

In  all  types  of  gas  engine  it  is  found  most  economical  to  keep 
the  motor  cylinders  as  hot  as  possible ;  they  are  generally  worked 
at  a  temperature  close  upon  the  temperature  of  boiling  water. 
This  is  done  to  diminish  the  loss  of  heat  from  the  explosion.  It 


The  Causes  of  Loss  in  Gas  Engines 


75 


follows  that  if  the  working  fluid  is  introduced  at  a  lower  temperature 
it  becomes  heated.  In  the  first  type,  the  charge  should  be  admitted 
and  remain  at  the  lowest  temperature  until  the  moment  of  explosion, 
which  is  of  course  impossible  if  the  cylinder  is  at  100°  C.  As  the 
piston  itself  is  hotter  than  that,  it  may  be  supposed  that  the  charge 
is  heated  to  that  point. 

Taking  an  extreme  case  and  calculating  the  effect  of  having 
an  absolute  temperature  of  390°  for  the  lower  limit,  it  will  be 
found  that  the  efficiency  is  diminished.  In  case  i,  type  i,  where 
the  expansion  is  carried  to  atmosphere  with  a  maximum  tem- 
perature of  1873°  absolute  =  1600°  C,  the  value  becomes  reduced 
to  0-23. 


With  a  maximum  temperature  of  1273°  absolute  =  1000°  C 
efficiency  is  o-i6. 

TYPE  I. 


the 


Initial  temp,  of  working 
fluid 

Max.  temp. 

Efficiency 

17°  C. 

1600°  C. 

O'29 

ii7°C. 

1600°  C. 

o'23 

17   C. 

icoo~  C. 

0*23 

n7aC. 

ioooj  C. 

o'i6 

Here  heating,  while  introducing  the  charge  will  always  cause 
diminution  in  efficiency,  the  proportion  of  loss  being  greater  with 
the  lower  maximum  temperature.  At  1600°  C.  the  loss  is  nearly 
one-fifth,  while  at  1000°  C.  it  is  close  upon  one-fourth. 

It  is  very  difficult  to  say  whether  it  is  better  to  work  with  the 
cylinder  hot  or  cold.  The  constructor  finds  himself  in  a  dilemma 
if  the  cylinder  is  kept  as  cold  as  the  surrounding  air  ;  then  the 
hot  gases  cool  more  rapidly.  If  he  keeps  the  cylinder  hot  to  diminish 
this,  the  efficiency  falls  also.  Experiment  alone  can  decide  the 
question. 

In  engines  of  type  2  it  is  a  usual  proceeding  to  leave  the 
compression  cylinder  entirely  without  water-jacketing,  under  the 
impression  that  heat  is  thereby  saved;  the  temperature  consequently 
rises  to  very  nearly  that  of  compression,  and  the  entering  charge 
becomes  considerably  heated  before  compression.  This  is  especi- 
ally the  case  if  the  admission  area  is  small,  and  throttling  occurs  ;  all 


76  The  Gas  Engine 

the  energy  of  velocity  of  the  entering  gas  becomes  transformed  into 
heat.  As  in  the  previous  case  the  charge  may  be  considered  to  rise 
to  117°  C.  before  compression. 

Where  expansion  is  carried  to  atmosphere  it  has  been  shown 
that  the  efficiency  is  quite  independent  of  the  maximum  tempera- 
ture, but  is  determined  by  one  circumstance  only  —  the  amount  of 
the  compression.  As 

•/i 

F  =  i  --  *  and  t  is  the  temperature  absolute  before  compressing 
*t 

'<•     »        »         »         »  atter          „ 

and  as  -c  '••=  [£]  ~,  it  follows  that  with  a  constant  ratio  between 


•        \p 

the  pressures  before  and  after  compression,  the  ratio  of  temperature 
before  and  after  compressing  will  also  remain  constant  ;  that  is, 
the  efficiency  is  not  in  any  way  affected  by  heating  the  working 
fluid,  provided  the  same  degree  of  compression  is  used.  Increase 
of  temperature  previous  to  compression  causes  a  proportional  in- 
crease of  temperature  after  compressing  without  in  any  way  disturb- 
ing the  ratio  between  them. 

This  is  an  important,  if  in  appearance  a  somewhat  paradoxical 
fact,  and  it  may  be  stated  in  another  way  : 

If  an  engine  receives  all  its  supply  of  heat  at  one  pressure, 
and  rejects  all  its  waste  heat  at  another  pressure,  after  falling 
from  the  higher  to  the  lower  pressure  by  expansion  doing  work, 
the  efficiency  is  constant  for  all  maximum  temperatures  of  the 
working  fluid. 

The  proportion  of  heat  converted  into  work  is  not  changed  in 
any  way  by  increasing  the  temperature  before  compressing,  and 
if  only  one  degree  of  heat  be  added  after  compressing,  the  same 
proportion  of  that  one  degree  is  converted  into  work,  as  would  be 
done  with  any  addition  of  heat  however  great. 

Where  the  expansion  is  not  continued  enough  to  reduce  the 
pressure  after  heating,  to  atmosphere,  as  in  the  cases  of  this  type 
which  occur  in  practice,  this  is  not  quite  true  ;  the  compression 
still  remains  the  most  powerful  element  of  efficiency,  but  heating 
before  compression  produces  some  change,  just  as  increase  of 
temperature  after  compression  produces  change.  The  change  is 

*  See  p.  57. 


The  Causes  of  Loss  in  Gas  Engines  77 

not  great,  and  it  is  always  in  the  direction  of  improvement  with  a 
limited  expansion.  If  the  lower  temperature  /  is  increased,  the 
compression  temperature  /,_  increases  in  proportion,  and  is  ac- 
cordingly nearer  the  maximum  temperature.  The  volume  increases 
less  on  heating,  so  that  the  effect  upon  efficiency  is  the  same  as  if 
the  expansion  had  been  increased  ;  the  terminal  pressure  will  more 
closely  approach  atmosphere,  and  therefore  come  nearer  to  the 
condition  of  maximum  efficiency. 

In  engines  of  type  3  the  compression  and  expansion  are  often 
performed  in  the  same  cylinder.  For  this  purpose  it  is  necessary 
to  leave  at  the  end  of  the  cylinder  a  space  into  which  the  charge 
is  to  be  compressed.  As  the  piston  does  not  enter  this  space,  a 
considerable  volume  of  exhaust  gases  remains  to  mix  with  the  fresh 
cold  charge.  Partly  from  this  and  partly  from  the  heating  effect 
of  the  cylinder  and  piston,  the  charge  becomes  considerably  heated 
before  compression.  The  temperature  of  200°  C.  is  not  unusual. 
Here  the  simplest  case  is  that  where  the  expansion  is  continued 
to  the  same  volume  as  existed  before  compression.  The  efficiency 
depends  solely  upon  the  amount  of  the  compression  ;  for  any 
given  degree  of  compression  it  is  constant,  whether  the  addition 
of  heat  at  constant  volume  after  compression  be  great  or  small. 

The   efficiency  is  E=I  —  '    as  in  type  2  (see  p.  57) ;  and  the  two 

absolute  temperatures  vary  in  the  same  ratio,  that  is,  if  the  charge 
is  heated  before  compression,  the  temperature  after  compression  will 
be  increased  in  the  same  ratio.  The  two  temperatures  will  therefore 
bear  a  constant  ratio  to  each  other,  whatever  the  initial  temperature 
may  be,  provided  the  compression  is  constant.  Heating  the  charge 
before  compression  will  consequently  have  no  disturbing  effect  upon 
the  theoretical  efficiency.* 

Where  the  expansion  is  carried  to  atmosphere  the  case  is 
different.  The  diagram  (fig.  15)  may  be  considered  to  be  made  up 
of  two  parts  giving  two  different  efficiencies,  the  sum  of  which  in 
this  case  is  0-51.  In  expanding  from  the  compression  volume  vc 
to  the  original  volume  r  (compression  75  Ibs.  per  square  inch) 

*  It  is  here  necessary  to  distinguish  between  theoretical  and  practical  efficiency. 
Heating  before  compression  diminishes  efficiency  in  practice  by  increasing  max- 
imum temperature,  and  therefore  loss  of  heat. 


78  The  Gas  Engine 

the  total  efficiency  is  0-37,  and  from  that  volume  to  v  and  atmos- 
pheric pressure,  o'i4.  The  latter  portion  still  obeys  the  same  law 
as  in  a  similar  case  of  type  i  ;  so  that  if  the  initial  temperature  at 
volume  v  be  supposed  117°  C.  it  will  lose  efficiency  in  a  similar 
way.  The  temperature  901°  C.  will  still  exist  at  that  point  of  the 
expanding  line,  so  that  it  may  be  taken  as  similar  to  the  case 
calculated  on  p.  75,  where  1000°  C.  is  the  maximum.  The  loss 
of  efficiency  there  is  from  0*23  to  o'i6  for  an  initial  temperature 
of  117°  C.,  which  makes  0*14  become  nearly  o-io.  The  total 
efficiency  would  therefore  be  0*47  instead  of  0-51  without  previous 
heating. 

Efficiency  diminishes  with  increased  temperature  of  working 
fluid  before  compressing,  if  the  expansion  is  carried  to  atmosphere, 
but  does  not  change  where  the  expansion  is  limited  to  the  initial 
volume. 

OTHER  CAUSES  OF  Loss. 

The  third,  fourth,  and  fifth  causes  of  loss  require  for  their  ex- 
amination a  comparison  of  the  actual  diagrams,  and  a  knowledge 
of  the  phenomena  of  explosion  and  combustion,  and  so  cannot  be 
discussed  at  this  stage. 


CHAPTER  V. 

COMBUSTION    AND    EXPLOSION. 

IN  the  preceding  chapters  the  gas  engine  has  been  considered 
simply  as  a  heat  engine  using  air  as  its  working  fluid  ;  it  has  been 
assumed  that  in  the  different  cycles,  the  engineer  is  able  to  give 
the  supply  of  heat  either  instantaneously,  or  slowly,  at  will  ;  and 
also  that  he  can  command  temperatures  so  high  as  1000°  C.  or 
1600°  C.  It  is  now  necessary  to  study  the  properties  of  gaseous 
explosive  mixtures  in  order  to  understand  how  far  these  assump- 
tions are  true. 

ON  TRUE  EXPLOSIVE  MIXTURES. 

When  an  inflammable  gas  is  mixed  with  oxygen  gas  in  certain 
proportions,  the  mixture  is  found  to  be  explosive  :  a  flame  ap- 
proached to  even  a  small  volume  contained  in  a  vessel  open  to  the 
air  will  produce  a  sharp  detonation.  Variation  of  the  proportions 
will  cause  change  in  the  sharpness  of  the  explosion.  There  is  a 
point  where  the  mixture  is  most  explosive  ;  at  that  point  the  in- 
flammable gas  and  the  oxygen  are  present  in  the  quantities 
requisite  for  complete  combination.  After  explosion  the  vessel 
will  contain  the  product  or  products  of  combustion  only,  no 
inflammable  gas  remaining  unconsumed,  or  oxygen  uncombined, 
both  having  quite  disappeared  in  forming  new  chemical  com- 
pounds. 

That  mixture  may  be  called  the  true  explosive  mixture. 

Definition. — When  an  inflammable  gas  is  mixed  with  oxygen 
in  the  proportion  required  for  the  complete  combination  of  both 
gases,  the  mixture  formed  is  the  true  explosive  mixture. 

If  the  chemical  formula  of  an  inflammable  gas  is  known,  the 
volume  of  oxygen  necessary  for  the  true  explosive  mixture  can 


8o  The  Gas  Engine 

beat  once  calculated.  Elementary  substances  combine  chemi- 
cally with  each  other  in  certain  weights  known  as  the  atomic  or 
combining  weights:  chemical  symbols  are  always  taken  as  repre- 
senting those  weights  of  the  elements  indicated.  In  dealing  with 
inflammable  gases  used  in  the  gas  engine  it  is  convenient  to 
remember  the  following  symbols  and  weights  : 


Element 

Symbol 

Combining  weight 

o 

16 

H 

N 

c 

Sulphur      

s 

s2 

In  entering  or  leaving  any  compound  the  elements  invariably 
enter  or  leave  in  weights  proportional  to  those  numbers  or 
multiples  of  them.  Thus  hydrogen  and  oxygen  combine  with 
each  other,  forming  water  ;  the  formula  of  the  compound  is 
H2O,  meaning  that  18  parts  by  weight  contain  16  parts  of  O  and 
2  parts  of  H.  Similarly  when  carbon  combines  with  oxygen  two 
compounds  may  be  formed,  according  to  the  conditions,  carbonic 
oxide  or  carbonic  acid,  formulae  CO  and  CO.2)  the  former  containing 
in  28  parts  by  weight,  12  parts  of  carbon  and  16  parts  of  oxygen  ; 
the  latter  in  44  parts  by  weight  containing  12  parts  of  carbon  and 
32  parts  of  oxygen. 

The  formula  of  a  compound  therefore  not  only  indicates  its 
nature  qualitatively,  but  it  also  indicates  its  quantitative  composition. 

H2O  not  only  tells  the  nature  of  water,  but  it  represents  18 
parts  by  weight ;  CO  means  28  parts  by  weight  of  carbonic  oxide  : 
CO2  means  44  parts  by  weight  of  carbonic  acid.  The  numbers  18, 
28  and  44  are  know  as  the  molecular  weights  of  the  three  com- 
pounds in  question. 

When  dealing  with  gases  it  is  more  convenient  to  think  in 
volumes  than  in  weights.  It  is  easier,  for  instance,  to  measure  the 
proportions  of  explosive  mixtures  by  volume  and  to  say  this  mix- 
ture contains  one  cubic  inch,  one  cubic  foot  or  one  volume  of 
inflammable  gas  to  so  many  cubic  inches,  feet  or  volumes  of  oxygen. 

Fortunately  there  exists  a  simple  relationship  between  the 
volumes  of  elementary  gases  and  their  combining  weights,  and 


Combustion  and  Explosion  8 1 

also  between  the  volumes  of  compounds  and   their  molecular 
weights. 

If  equal  volumes  of  the  elementary  gases  are  weighed,  under 
similar  conditions  of  temperature  and  pressure,  it  is  found  that 
their  weights  are  proportional  to  the  combining  weights.  Taking 
the  weight  of  the  hydrogen  as  i,  then  the  weights  of  equal 
volumes  of  nitrogen  and  oxygen  are  14  and  16  respectively.  If 
then  it  is  wished  to  make  a  mixture  of  hydrogen  and  oxygen  gases 
in  the  proportion  of  2  parts  by  weight  of  the  former  to  16  parts  by 
weight  of  the  latter,  it  is  only  necessary  to  take  2  vols.  H  and 

1  vol.  O.     The  law  may  be  stated  in  two  ways,  as  follows  : 

Taking  hydrogen  as  unity  the  specific  gravity  of  the  elementary 
gases  is  the  same  as  their  combining  weights  ;  or 

The  combining  volumes  of  the  elementary  gases  are  equal. 

Instead  of  troubling  to  weigh  out  portions  of  the  gases  it  is 
at  once  known  that  one  volume  of  nitrogen  weighs  14  parts,  the 
same  volume  of  hydrogen  weighing  one  part,  oxygen  16  parts,  and 
so  on  through  all  the  gaseous  elements,  under  the  same  tempera- 
tures and  pressures. 

Knowing  that  water  is  the  compound  formed  by  the  ccmbus- 
tion  of  hydrogen  and  oxygen,  and  that  its  formula  is  H2O,  it  is  at 
once  apparent  that  the  true  explosive  mixture  of  these  gases  is 

2  vols.  H  and  i  vol.  O.     By  experiment  it  is  found  that  the  volrm  j 
of  the  water  produced  is  less  (of  course  in  the  gaseous  state)  than 
the  volume  of  the  mixed  gases  before  combination. 

The  measurement  requires  to  be  made  at  a  temperature  high 
enough  to  keep  the  steam  formed  in  the  gaseous  state.  Measure 
2  vols.  H  and  i  vol.  O  into  a  strong  glass  vessel  heated  to  130°  C. ; 
the  total  is  3  vols.  ;  fire  by  the  electric  spark  over  mercury.  It 
will  be  found  that  the  steam  formed  when  it  has  cooled  to  130°  C. 
after  the  explosion,  measures  2  vols.  It  has  been  found  to  be 
true  for  all  gaseous  compounds,  that  however  many  volumes  of 
elementary  gases  combine  to  form  them  the  product  is  always  two 
volumes.  In  elementary  gases,  one  volume  always  contains  the 
combining  weight  ;  in  compound  gases,  two  volumes  always  con- 
tain the  molecular  weight.  Compared  with  hydrogen,  therefore, 
the  specific  gravity  of  a  gaseous  compound  is  always  one-halt"  of 
the  molecular  weight. 


82 


The  Gas  Engine 


As  before,  the  law  may  be  stated  in  two  ways  : 

Taking  hydrogen  as  unity,  the  specific  gravity  of  a  compound 
gas  is  half  its  molecular  weight ;  or 

The  combining  volume  of  a  compound  gas  is  always  equal  to 
double  that  of  an  elementary  gas. 

These  laws  are  known  as  Gay-Lussac's  laws,  and  form  part  of 
the  very  basis  of  modern  chemistry. 

Using  them,  the  true  explosive  mixtures  by  volume  and  the 
volumes  of  the  products  of  the  combination  can  be  found  for  any 
gas  or  mixture  of  gases,  whether  elementary  or  compound. 

The  inflammable  compound  gases,  used  in  the  gas  engine, 
forming  some  of  the  constituents  of  coal  gas  are  : 


Inflammable  gas 

Formula 

Molecular  weight 

Molecular  vol. 

Marsh  gas  .... 
Ethylene  .... 
Carbonic  oxide  . 

CH4 
C9H4 
CO 

16 
28 
28 

2 
2 
2 

Applying  Gay-Lussac's  laws,  the  oxygen  required  for  true 
explosive  mixtures  and  the  volumes  of  the  products  of  combus- 
tion are  as  follows  for  all  the  inflammable  gases  used  in  the  gas 


engine  : 

2vols.  hydrogen  (H)  require  i  vol.  oxygen  (O)  forming    . 
2  vols.  marsh  gas  (CH4)  require  4  vols.  oxygen  (O)  forming 
2  vols.  ethylene  (C2H4)  require  6  vols.  oxygen  (O)  forming 


H.O 

Steam. 

2  vols. 
4  vols. 
4  vols. 


2  vo  s.  carbonic  oxide  (CO)  require  i  vol.  oxygen  (O)  forming         — 
2  vols.  tetrylene  (C4H8)  require  12  vols.  oxygen  (O)  forming    .     8  vols. 


CO, 

Carbonic 
acid. 

2  vols. 
4  vols. 
2  vols. 
8  vols. 


With  hydrogen  and  oxygen  3  volumes  before  combination 
become  2  volumes  after  combination.  CH4  and  O,  also  C2H4  and 
O,  the  volumes  of  the  products  of  combustion,  are  equal  to  the 
volumes  of  mixture.  With  carbonic  oxide  and  oxygen  3  volumes 
before  become  2  volumes  after  combination. 

ON  INFLAMMABILITY. 

Previous  to  1817,  Sir  Humphry  Davy  made  the  admirable 
researches  which  led  him  to  the  invention  of  the  safety  lamp.  He 
then  made  experiments  upon  different  explosive  mixtures,  and 
found  that  under  certain  conditions  they  lost  the  capability  of 


Combustion  and  Explosion  83 

ignition  by  the  electric  spark.  True  explosive  mixtures,  he  ob- 
served, may  lose  inflammability  in  two  ways  ;  by  the  addition  of 
excess  of  either  ol  the  gases  or  of  any  inert  gas  such  as  nitrogen, 
and  by  rarefaction.  The  hydrogen  explosive  mixture,  if  reduced  to 
one-eighteenth  of  ordinary  atmospheric  pressure,  cannot  be  in- 
flamed by  the  spark.  Heated  to  dull  redness  at  this  pressure  it 
will  recover  its  inflammability  and  the  spark  will  cause  combination. 

One  volume  of  the  mixture  to  which  has  been  added  nine 
volumes  of  oxygen  is  uninflammable,  but  if  the  density  is  increased 
or  the  temperature  raised,  it  recovers  its  inflammability. 

Eight  volumes  of  hydrogen  added,  produces  the  same  effect  as 
the  nine  volumes  of  oxygen,  but  only  one  volume  of  marsh  gas 
or  half  a  volume  of  ethylene  is  required.  The  excess  which  destroys 
inflammability  varies  with  the  temperature,  increasing  with  increase 
of  temperature.  Heating  the  mixture  widens  the  range,  both  of 
dilution  with  excess  or  inert  gas  and  reduction  of  pressure. 

The  point  where  inflammability  ceases  by  diluting  is  very 
abrupt  and. sharply  defined.  The  author  has  found  that  a  coal 
gas  which  will  inflame  by  the  spark  in  a  mixture  of  i  gas  and 
14  air  will  not  inflame  with  15  of  air.  If  the  experiment  be  re- 
peated on  a  warmer  day  it  may  inflame  with  15  of  air,  but  will  not 
with  1 6  air.  As  the  proportion  is  fixed  for  any  given  temperature 
it  will  be  convenient  to  call  that  proportion  for  any  mixture  the 
'  critical  proportion.'  Any  mixture  in  the  critical  proportion  be- 
comes inflammable  by  a  very  small  increase  of  temperature  or 
pressure.  The  exact  limits  of  dilution  temperature  and  pressure 
have  yet  to  be  discovered. 

Passing  from  any  true  explosive  mixture  by  dilution  to  the 
mixture  in  the  critical  proportion,  the  inflammability  slowly 
diminishes,  the  explosion  becoming  less  and  less  violent,  till  at  last 
no  report  whatever  is  produced,  and  the  progress  of  the  flame  (if 
a  glass  tube  is  used)  is  easily  followed  by  the  eye. 

In  his  great  work  on  gas  analysis,  Professor  Bunsen  confirms 
Davy's  observations  in  every  particular,  proving  loss  of  inflam- 
mability by  dilution  and  reduction  of  pressure  as  well  as  its 
restoration  by  heating,  increase  of  pressure  and  slight  addition  of 
the  inflammable  gas.  His  work,  however,  was  not  published  till 
1857- 

G  2 


84  TJie  Gas  Engine 


ON  THE  RATE  OF  FLAME- PROPAGATION. 

The  sharp  explosion  of  a  true  explosive  mixture  is  due  to  the 
very  rapid  rate  at  which  a  flame,  initiated  at  one  point,  travels 
through  the  entire  miss  and  thereby  causes  the  maximum  pressure 
to  be  rapidly  attained.  With  a  diluted  mixture  the  flame  travels 
more  slowly.  Dilution  therefore  diminishes  explosiveness  in  two 
ways — by  increasing  the  time  of  getting  the  highest  pressure  and 
also  by  diminishing  the  highest  pressure  which  can  be  got 
Professor  Bunsen's  experiments  are  the  earliest  attempts  to 
measure  the  velocity  of  flame  movement  in  explosive  mixtures. 
His  method  is  as  follows  : 

The  explosive  mixture  is  allowed  to  burn  from  a  fine  orifice  of 
known  diameter,  and  the  rate  of  the  current  of  the  issuing  gas 
carefully  regulated  by  diminishing  the  pressure  to  the  point  at 
which  the  flame  passes  back  through  the  orifice  and  inflames  the 
explosive  mixture  below  it  This  passing  back  of  the  flame  occurs 
when  the  velocity  with  which  the  gaseous  mixtures  issue  from  the 
orifice  is  inappreciably  less  than  the  velocity  with  which  the  in- 
flammation of  the  upper  layers  of  burning  gas  is  propagated  to  the 
lower  and  unignited  layers.  Knowing  then  the  volume  of  mixture 
passing  through  the  orifice  and  its  diameter,  the  rate  of  flow  at 
the  moment  of  back  ignition  is  known.  It  is  identical  with  the  rate 
of  flame  propagation  through  the  mixture. 

Bunsen  made  determinations  for  the  true  explosive  mixtures 
of  hydrogen  and  carbonic  oxide. 

VELOCITY  OF  FLAME  IN  TRUE  EXPLOSIVE  MIXTURES.     (Bunsen.) 

Hydrogen  mixture  (2  vols.  H  and  i  vol.  O) .         -34  metres  per  sec. 
Carbonic  oxide  mixture  (i  vol.  tO  and  i  vol.  O)  .       i  metre  per  sec.  nearly. 

The  method  is  a  singularly  simple  and  beautiful  one  and 
answered  thoroughly  for  Professor  Bunsen's  purpose  at  the  time 
he  devised  it.  Several  objections,  however,  may  be  brought  against 
it.  The  mixture  in  issuing  from  the  jet  into  the  air  as  flame, 
becomes  mixed  to  some  extent  with  the  air  and  so  cools  down  ; 
the  metal  plate  also,  pierced  with  the  orifice,  exercises  a  great 
cooling  effect.  If  the  hole  were  made  small  enough  the  flame 
could  not  pass  back  at  all,  however  much  the  flow  is  reduced, 


Combustion  and  Explosion  85 

because  the  heat  would  be  conducted  away  so  rapidly  as  to 
extinguish  the  flame.  This  had  been  shown  by  Davy  in  1817; 
indeed  it  is  the  principle  of  the  safety  lamp.  These  causes  prob- 
ably make  Bunsen's  velocities  too  low.  MM.  Mallard  and  Le 
Chatelier  have  made  velocity  determinations  by  a  method  designed 
to  obviate  those  sources  of  error. 

The  explosive  mixture  is  contained  in  a  long  tube  of  considerable 
diameter,  closed  at  one  end,  open  to  the  atmosphere  at  the  other. 
At  each  end  a  short  rubber  tube  terminates  in  a  cylindrical  space 
closed  by  a  flexible  diaphragm.  A  light  style  is  fixed  upon  the 
diaphragms.  A  drum  revolves  close  to  each  style,  both  drums 
upon  the  same  shaft.  A  tuning  fork,  vibrating  while  the  experi- 
ment is  being  made,  traces  a  sinuous  line  upon  the  drum  and  so 
the  rate  of  revolution  is  known.  The  mixture  is  ignited  at  the 
open  end,  and  the  flame  in  passing  the  lateral  opening  leading  to 
the  first  diaphragm  ignites  the  mixture  there,  and  so  moves  the 
style  and  marks  the  drum ;  the  arrival  of  the  flame  is  signalled  at 
the  other  end  in  the  same  way.  The  drums  revolving  together, 
the  distance  between  the  two  style  markings  measured  by  the 
vibration  marks  of  the  tuning  fork  gives  the  time  taken  by  the 
flame  to  move  between  the  two  points.  The  numbers  got  in  this 
way  are  the  rates  of  the  communication  of  the  flame  through  the 
mixture,  back  into  the  tube,  while  the  flame  can  freely  expand  to  the 
air;  when  both  ends  are  closed  the  velocity  is  much  greater.  Then, 
not  only  does  the  flame  spread  from  particle  to  particle  of  the 
explosive  mixture  at  the  rate  due  to  contact  of  the  inflamed  particles 
with  the  uninfiamedones,  but  the  expansion  produced  by  the  inflam- 
mation projects  the  flame  mechanically  into  the  other  part  and  ?o 
produces  an  ignition,  which  does  not  travel  at  a  uniform  rate,  but  at  a 
continually  accelerating  one.  In  the  same  way,  using  the  open  tube 
but  firing  at  the  closed  end,  the  expansion  of  the  first  portion  adds 
to  the  apparent  velocity  of  propagation,  and  projects  the  last 
portion  of  the  mixture  into  the  atmosphere.  The  true  velocity  of 
the  propagation  is  the  rate  at  which  the  flame  proceeds  from  particle 
of  inflamed  mixture  to  uninflamed  particle  by  simple  contact  ;  the 
true  velocity  depends  upon  inflammability  alone,  the  rate  under 
other  conditions  depends  also  upon  heat  evolved,  and  therefore 
movement  due  to  expansion,  mechanical  disturbance  of  the  unig- 


86  TJie  Gas  Engine 

nited  by  the  projection  of  the  ignited  portion  into  its  midst.    These 
conditions  may  vary  much  ;  the  inflammability  remains  constant. 

Mallard  and  Le  Chatelier's  results  for  the  true  velocity  of  pro- 
pagations are  : 

VELOCITY  OF  FLAME  IN  TRUE  EXPLOSIVE  MIXTURES. 
(Mallard  and  Le  Chatelier.) 

per  sec. 

Hydrogen  mixture  (2  vols.  H  and  i  vol.  O)   .         .         .20  metres. 
Carbonic  oxide  (2  vols.  CO  and  i  vol.  O)      .        .        .     2  "2     ,, 

Bunsen's  rate  for  hydrogen  mixture  seems  to  have  been  too 
great,  and  for  carbonic  oxide  mixture  too  little.  The  rate  for  a  true 
and  very  explosive  mixture  such  as  hydrogen  is  liable  to  be  inac- 
curately determined,  as  temperature  variation  makes  a  great 
change,  and  it  is  difficult  even  with  Mallard  and  Le  Chatelier's 
method  to  obtain  concordant  experiments.  With  less  inflammable 
mixtures  the  difficulty  disappears.  As  true  explosive  mixtures  are 
never  used  in  the  gas  engine,  their  properties  concern  the  engineer 
only  as  a  preliminary  to  the  study  of  diluted  mixtures.  The  most 
explosive  mixture  which  can  be  made  with  air  contains  a  large 
volume  of  nitrogen  inevitably  present  as  diluent. 

The  following  are  some  of  their  results  with  diluted  mixtures, 
which  are  stated  to  be  correct  within  10  per  cent,  error  of  experi- 
ment : 
VELOCITY  OF  FLAME  IN  DILUTED  MIXTURES.     (Mallard  and  Le  Chatelier. ) 

per  sec. 

i  vol.  hydrogen  mixture  +  J  vol.  oxygen  .        .     17  -3  metres. 

,,  ,,  +i  vol.  oxygen 


+  i  vol.  hydrogen 
+  i  vol.  hydrogen 


18 
11-9 


,,  ,,  +2  vols.  hydrogen 

These  rates  show  that  the  true  explosive  mixture  of  hydrogen 
and  oxygen  when  diluted  with  its  own  volume  of  oxygen  falls  from 
20  metres  per  second  to  10  metres,  that  is,  it  becomes  one-half 
as  inflammable  ;  when  its  own  volume  of  hydrogen  is  the  diluent, 
the  velocity  only  falls  to  11*9  metres  per  second.  Hydrogen  there- 
fore has  less  effect  in  diminishing  inflammability  than  oxygen. 

Remembering  the  fact  that  the  atmosphere  contains  one- fifth 
of  its  volume  of  oxygen,  the  remaining  four-fifths  being  nearly  all 
nitrogen,  it  is  easy  to  get  the  proportions  for  the  strongest  explosive 


Combustion  and  Explosion 


mixture  possible  with  air.    Two  volumes  hydrogen  require  i  volume 
oxygen,  and  therefore  5  volumes  air.  The  strongest  possible  mixture 
with  air  is  two-sevenths  hydrogen,  five-sevenths  air.  The  following 
experiments  are  for  hydrogen  and  air  in  different  proportions  : 
VELOCITY  OF  FLAME  IN  DILUTED  MIXTURES.     (Mallard  and  Le  Cfiatelier.) 


Mixture, 


vol.  H  and  4  vols.  air 
H  and  3  vols.  air 
H  and  2^  vols.  air 
H  and  if  vols.  air 
H  and  i£  vols.  air 
H  and  i  vol.  air 
H  and  i  vol.  air 


per  sec 

2  metres. 
2-8 

3  "4 
4'i 
4 '4 

2 '3 


Very  strangely  the  velocity  is  greatest  when  there  is  an  excess 
of  hydrogen  present.  To  get  just  enough  of  oxygen  for  complete 
burning  i  volume  H  requires  2.7  volumes  air,  which  would  be 
naturally  supposed  to  be  the  most  inflammable  mixture,  as  it  gives 
out  the  greatest  heat,  but  for  some  reason  it  is  not.  When  the 
hydrogen  is  increased  beyond  that  point  the  velocity  again  falls  off. 
A  determination  for  coal  gas  and  air  gave  i  volume  gas,  5 
volumes  air  a  velocity  of  i'oi  metres  per  second,  and  i  volume  gas, 
6  volumes  air  0-285  metres  per  second.  With  coal  gas  also  the 
maximum  velocity  is  got  with  the  gas  slightly  in  excess. 

So  far,  these  rates  of  ignition  or  inflammation  are  measures  of 
inflammability,  and  are  the  rates  for  constant  pressure;  the  rates  for 
constant  volume  are  very  different,  and  the  problem  is  a  more 
complex  one.  Inflaming  at  the  closed  end  of  the  tube,  they  found 
that  even  very  dilute  mixtures  gave  a  sharp  explosion,  and  in  the 
case  of  hydrogen  true  explosive  mixture,  the  velocity  became  1000 
metres  per  second  instead  of  20.  With  hydrogen  and  air  300 
metres  per  second  were  obtained. 

MM.  Berthelot  and  Vieille  have  proved  that  under  certain  con- 
ditions even  greater  velocities  than  these  are  possible.  The  con- 
ditions, however,  are  abnormal,  and  the  generation  of  M.  Berthelot's 
explosive  wave  is  exceedingly  undesirable  in  a  gas  engine.  It  is 
generated  by  inflaming  a  considerable  portion  of  the  mixture  at 
once,  and  so  causing  the  transmission  of  a  shock  from  molecule 
to  molecule  of  the  uninflamed  mixture:  this  shock  causes  an 
ignition  velocity  nearly  as  rapid  as  the  actual  mean  velocity  of 
movement  of  the  gaseous  molecules  at  the  high  temperatures,  of 


. 
88  The  Gas  Engine 

combustion.  The  difference  between  this  almost  instantaneous 
detonation  and  the  ordinary  flame  propagation  may  be  compared 
to  similar  differences  in  the  explosion  of  gun  cotton  discovered 
by  Sir  Frederic  Abel.  Gun  cotton  lying  loosely,  and  open  to  the 
air,  will  burn  harmlessly  if  ignited  by  a  flame;  indeed,  a  consider- 
able portion  may  be  laid  upon  the  open  hand  and  ignited  by  a 
flame  without  the  smallest  danger.  The  same  quantity  in  the 
same  position,  if  fired  by  a  percussive  detonator,  will  occasion 
the  most  violent  explosion,  the  nature  of  the  shock  given  to  the 
gun  cotton  by  the  detonator  causing  a  transmission  of  the  kind  of 
vibration  necessary  to  cause  its  almost  instantaneous  resolution 
into  its  component  gases. 

The  explosive  wave  in  gases  seems  to  originate  in  like  con- 
ditions. Its  velocity  for  the  true  explosive  mixture  of  hydrogen 
and  oxygen  is  2841  metres  per  second,  and  for  carbonic  oxide 
mixture,  1089  metres  per  second.  The  velocity  is  independent  of 
pressure  between  half  an  atmosphere  and  one  and  a  half  atmo- 
sphere. It  is  independent,  too,  of  the  diameter  of  the  tube  used, 
within  considerable  limits,  or  of  the  material  of  the  tube,  rubber 
and  lead  tubes  giving  similar  results.  Diluting  the  mixtures  di- 
minishes, and  heating  increases  it.  The  experiments  are  very 
interesting  and  important,  from  a  physicist's  standpoint,  but, 
fortunately  for  the  inventor  dealing  with  gas  engines,  the  explosive 
wave  is  not  easily  generated  in  a  gas  engine  cylinder;  if  it  were, 
it  would  be  impossible  to  run  the  engines  without  shock  and 
hammering. 

The  velocity  which  really  concerns  the  engineer  is  that  due 
to  inflammability;  and  expansion  produced  by  inflaming — the 
velocity,  in  fact,  with  which  the  inflammation  spreads  through  a 
closed  vessel.  As  it  cannot  be  discussed  without  considering 
other  matters — heat  evolved  by  combustion,  and  temperatures  and 
pressures  produced — it  will  be  advisable  first  to  give  the  heat 
.  evolved  by  combustion,  and  then  devote  a  complete  chapter  to 
explosion  in  a  closed  vessel. 

HEAT  EVOLVED  BY  COMBUSTION. 

Careful  experiments  upon  the  heat  evolved  by  the  combustion 
of  gases  in  oxygen  have  been  made  by  Favre  and  Silberman,  and 


Combustion  and  Explosion  89 

also  by  Professor  Andrews.  The  physicists  first  named  burned 
the  gases  at  constant  pressure  in  a  specially  devised  calorimeter. 
Professor  Andrews  mixed  the  gases  in  a  thin  spherical  copper 
vessel,  closed  it,  and  exploded  by  the  spark:  the  vessel  being  sur- 
rounded by  water  gave  up  its  heat  to  the  water,  the  weight  of  which 
being  known,  the  rise  of  temperature  gave  the  heat  evolved. 

Quantities  of  heat  are  measured  by  taking  water  as  the '  unit. 
In  this  work,  a  heat  unit  always  means  the  amount  of  heat  neces- 
sary to  raise  unit  weight  of  water  through  i°  C. 

Taking  an  average  of  Favre  and  Silberman  and  Andrews's 
results,  the  inflammable  gases  used  in  gas  engines  evolve  upon  com- 
plete combustion  the  following  amounts  of  heat  : 

Heat  units. 

Unit  weight  of  hydrogen  completed  burned  to  H2O  evolves  .  .  34,170 
Unit  weight  of  carbon  completely  burned  to  CO.>  evolves  .  .  .  8,000 
Unit  weight  of  carbonic  oxide  completely  burned  to  COo  evolves  .  2,400 
Unit  weight  of  marsh  gas  completely  burned  to  CO,  and  FLO  evolves  13,080 
Unit  weight  of  ethylene  completely  burned  to  CO2  and  H.,,0  evolves  11,900 

That  is,  one  pound  weight  of  hydrogen  burned  completely  to 
w.iter  will  evolve  as  much  heat  as  would  raise  34,170  Ibs.  of  water 
through  i°  C.,  or  the  converse.  One  pound  of  carbon  in  burning 
to  carbonic  acid  evolves  as  much  heat  as  would  raise  8,000  Ibs.  of 
water  through  i°  C.  These  numbers  give  the  amount  or  quantity 
of  heat  evolved.  The  intensity  or  temperature  of  the  combustion 
may  be  calculated  on  the  assumption  that  the  whole  heat  is  evolved 
under  such  conditions  that  no  heat  is  lost,  or  is  applied  to  any- 
thing else  but  the  products  of  combustion.  To  make  the  calcu- 
lation it  is  necessary  to  know  the  specific  heat  of  the  products. 

The  amount  of  heat  required  to  heat  unit  weight  of  water 
through  one  degree  is  i  heat  unit,  the  specific  heat  of  any  other 
body  is  the  number  of  heat  units  required  to  heat  unit  weight  of  the 
body  through  one  degree.  Gases  have  two  different  specific  heats 
depending  upon  whether  heat  is  applied  while  the  gas  is  kept  at  con- 
stant volume,  oral  constant  pressure;  both  are  required  in  dealing 
with  gas  engine  problems.  The  specific  heat  at  constant  volume 
is  sometimes  known  as  the  true  specific  heat;  in  taking  the  specific 
heat  at  constant  pressure  the  gas  necessarily  expands,  and  so  does 
work  on  the  external  air;  this  specific  heat  is  therefore  greater 
than  the  former  by  the  amount  of  work  done.  For  the  gases  used 


90 


The  Gas  Engine 


in  the  gas  engine  the  two  values  are  as  follows.  The  ratio  be- 
tween the  two  is  also  given,  as  it  is  frequently  required  in  efficiency 
calculations.  The  experimental  numbers  are  Regnault's,  the 
calculated  specific  heat  at  constant  volume,  Clausius. 

SPECIFIC  HEATS  OF  GASF.S. 
(For  equal  -weigh! s.      Water  =  i.) 


Name  of  gas 

Sp.  heat  at 
constant  pressure 

Sp.  heat  at 
constant  volume 

Sp.  heat  con.  pres. 
Sp.  heat  con.  vol. 

Air 

0-237 

O'i68 

HIS 

Oxygen 

0*217 

0-155 

•403 

Nnrogen 

0-244 

0-173 

•409 

Hydrogen 

3  '409 

2  '406 

HI? 

Marsh  gas        • 

°'593 

0-467 

— 

Ethylene 

0*404 

0-332 

•144 

Carbonic  oxide 

0-245 

0-173 

•416 

Steam    . 

0-480 

0-369 

1-302 

Carbonic  acid 

O"2l6 

0-I7I 

1-165 

It  is  convenient  to  remember  that  the  specific  heats  of  com- 
bining or  atomic  weights  of  the  elements  are  equal  —  Dulong  and 
Petit's  law.  To  this  law  there  are  few  exceptions,  and  the  per- 
manent elementary  gases,  oxygen,  nitrogen,  and  hydrogen,  obey  it 
almost  absolutely.  As  equal  volumes  of  these  gases  represent  the 
combining  weights,  it  follows  that  equal  volumes  of  these  gases 
have  the  same  specific  heat.  Taking  the  specific  heat  of  air  as 
the  unit,  the  specific  heat  of  hydrogen  and  oxygen  gases  is  also 
unity.  The  compound  gases  do  not  obey  the  law  so  closely. 
The  calculation  of  temperature  of  combustion  can  now  be  made. 
The  amount  of  heat  evolved  from  unit  weight  of  a  combustible  is 
usually  said  to  measure  its  calorific  power,  that  amount  divided  by 
the  specific  heat  of  the  products  of  the  combustion  is  said  to  be  the 
measure  of  its  calorific  intensity.  The  calorific  intensity  is  indeed 
the  theoretical  temperature  of  the  combustion  :  taking  hydrogen 
first,  unit  weight  evolves  34,170  heat  units.  But  the  water  formed 
weighs  9  units  (from  formula  H2O),  and  if  its  specific  heat  in  the 
gaseous  state  were  unity,  the  supposed  maximum  temperature  of 


combustion  would  be  =  3796-6.     But  the  specific  heat  is 


Combustion  and  Explosion  91 

less  than  unity  ;  therefore  the  theoretical  maximum  will  be  greater. 

It  is — 341/0 —  =   79007.     For  certain  reasons  to  be  considered 
9  x  0-480 

later,  no  such  enormous  temperatures  are  ever  attained  by  com- 
bustion. In  the  above  calculation  the  latent  heat  of  steam  should 
first  have  been  deducted,  as  it  is  included  in  the  total  heat  evolved 
as  measured  by  the  calorimeter  :  it  is  537  heat  units.  34,170  —  537 
gives  the  total  heat  available  for  increasing  the  temperature,  the 

amended  calculation  is  34I7°  ~  537  =7 7 85 -4,  still  an  exceedingly 

high  temperature. 

Calculating  the  heat  evolved  by  burning  carbon  in  the  ?ame 
way,  but  omitting  any  deduction  for  the  latent  heat  of  carbonic 
acid  (it  does  not  affect  the  calorimeter,  as  it  does  not  condense), 
the  theoretical  temperature  produced  by  burning  in  oxygen  is 
still  higher,  being  10,174°  C.  Burning  in  air  the  theoretical 
temperatures  are  lower  as  the  nitrogen  present  acts  as  a  diluent, 
and  must  necessarily  be  heated  to  the  same  temperature  as  the 
products  of  the  combustion.  They  are  given  as  follows  in  '  Watts' 
Dictionary. ' 

Temperature  produced 

Calorific  power          v 

In  oxygen          In  air 

Carbon          ....  8080          10174°  C.  271      C. 

Hydrogen      ....         34462  6930°  C.  2741°  C. 

These  are  the  supposed  temperatures  burning  in  the  open 
atmosphere,  and  therefore  at  constant  pressure,  the  gases  expand- 
ing doing  work  upon  the  air.  At  constant  volume,  that  is, 
burning  in  a  closed  vessel  so  that  the  volume  cannot  increase  but 
only  the  pressure,  the  temperature  should  be  greater  as  the 
specific  heat  at  constant  volume  is  less.  Allowing  for  that,  the 
numbers  become 

THEORETICAL  TEMPS.  OF  COMBUSTION  AT  CONSTANT  VOLUME. 

Temperature  produced 

In  oxygen       In  air 

Carbon       ....        12820 
Hydrogen  ....          9010  4119 

Such  temperatures  have  never  been  produced  by  combustion, 


92  The  Gas  Engine 

for  many  reasons,  of  which  all  save  the  most  potent  have  been 
discussed  by  the  earlier  writers  on  heat     This  is  Dissociation. 


DISSOCIATION. 

Most  chemical  combinations,  while  in  the  act  of  formation 
from  their  constituent  elements,  evolve  heat,  and  as  a  general 
rule,  the  greater  the  heat  evolved  the  more  stable  is  the  com- 
pound formed.  The  compound  after  formation  may  generally  be 
decomposed  by  heating  to  a  high  enough  temperature,  heat  being 
one  of  the  most  powerful  splitting  up  agencies  known  to  the  chemist. 
The  nature  of  the  decomposition  varies  with  the  compound.  In 
many  cases  the  process  is  irreversible,  that  is,  although  heating  up 
will  cause  decomposition,  cooling  down  again,  however  slowly, 
will  not  cause  recombination.  In  some  compounds,  however,  under 
certain  conditions  the  process  is  reversible,  and  recombination 
occurs  on  slow  cooling. 

Definition.  —  Dissociation  may  be  defined  as  a  chemical 
decomposition  by  the  agency  of  heat,  occurring  under  such  con- 
ditions that  upon  lowering  the  temperature  the  constituents 
recombine. 

Groves  found  long  ago  that  water  begins  to  split  up  into 
oxygen  and  hydrogen  gases  at  temperatures  low  compared  to  that 
produced  by  combustion.  Deville  made  a  careful  study  of  the 
phenomena,  and  found  that  decomposition  commences  at  960°  to 
1000°  C.  and  proceeds  to  a  limited  extent  :  raising  the  temperature 
to  1200°  C.  increases  it,  but  a  limit  is  reached.  The  amount  of 
decomposition  depending  upon  the  temperature,  for  each  tempe- 
rature there  is  a  certain  proportion  between  the  amount  of  steam 
and  the  amount  of  free  oxygen  and  hydrogen  gases  present.  If 
the  temperature  is  increased,  the  proportion  of  free  gases  also 
increases  :  if  temperature  is  diminished,  the  proportion  of  free 
gases  diminishes.  If  the  temperature  be  raised  beyond  a  certain 
intensity,  the  water  is  completely  decomposed  :  if  lowered  beyond 
a  certain  temperature,  complete  combination  results.  The  same 
thing  happens  with  carbonic  acid, -the  temperature  of  decomposition 
is  lower. 

It  is  quite  evident,  then,  that  at  the  highest  temperatures  pro- 


Combustion  and  Explosion  93 

duced  by  combustion,  the  product  cannot  exist  in  the  state  of 
complete  combination.  It  will  be  mixed  to  a  certain  extent  wiih 
the  free  constituents  which  cannot  combine  further  until  the  tem- 
perature falls;  as  the  temperature  falls,  combustion  will  continue 
till  all  the  free  gases  are  combined.  The  subject,  from  its  nature, 
is  a  difficult  one  in  experiment,  and  accordingly  different  observers 
do  not  quite  agree  upon  temperatures  and  percentages  of  dissocia- 
tion, but  all  are  agreed  that  dissociation  places  a  rigid  barrier  in 
the  way  of  combustion  at  high  temperatures,  and  prevents  the 
attainment  of  temperatures,  by  combustion,  which  are  otherwise 
quite  possible.  With  no  dissociation,  hydrogen  burning  in  oxygen 
should  be  able  under  favourable  circumstances  to  give  a  tempera- 
ture of  over  6000°  C,  as  has  been  shown.  Deville's  experiments 
upon  the  temperature  of  the  oxyhydrogen  flame,  at  constant 
pressure  of  the  atmosphere,  gave  under  2500°  C.  The  estimate 
was  made  by  melting  platinum  in  a  lime  crucible,  with  the  oxy- 
hydrogen flame  playing  upon  the  platinum,  the  crucible  being 
well  protected  against  loss  of  heat  by  lime  blocks,  so  that  the 
platinum  could  really  attain  the  temperature  of  the  flame;  when 
at  the  highest  temperature,  the  molten  platinum  was  rapidly 
poured  into  a  weighed  calorimeter,  and  the  rise  in  temperature 
noted.  From  this  was  calculated  the  temperature  of  the  platinum. 
The  experiment  was  dangerous  and  inaccurate,  but  it  is  the  only 
serious  attempt  which  has  been  made  to  determine  the  temperature 
of  the  oxyhydrogen  flame  at  constant  pressure. 

The  highest  temperature  produced  by  hydrogen  burning  in 
oxygen  has  been  determined  by  Bunsen,  and  also  Mallard  and 
Le  Chatelier,  for  combustion  at  constant  volume,  that  is,  ex- 
plosioa 

As  the  theoretic  calculation  shows,  with  no  dissociation  a 
temperature  of  9000°  C.  is  possible.  The  highest  maximum  it 
is  possible  to  assume  from  Bunsen's  experiments  is  3800°  C.  ; 
from  Mallard  and  Le  Chatelier's,  3500°  C.  The  two  sets  of  ex- 
periments are  concordant.  It  is  true  the  latter  physicists  do  not 
attribute  the  difference  wholly  to  dissociation,  but  they  agree  that 
part  is  due  to  this  cause;  and  that  there  is  an  enormous  difference 
between  heat  temperature  actually  got  and  that  which  should  be 
possible  if  no  limit  existed  all  are  agreed.  With  air,  Bunsen's 


94  The  Gas  Engine 

figures  show  a  maximum  of  about  2000°  C,  Mallard  and  Le 
Chatelier  say  1830°  C.;  the  present  writer  has  also  made  experi- 
ments with  hydrogen  in  air,  and  finds  the  highest  possible  tem- 
perature to  be  1900°  C.  The  calculated  maximum  is  3800°  C. 
The  difference  is  not  so  great  as  with  the  true  explosive  mixture, 
which  is  to  be  expected,  but  all  experiments  agree  in  proving  that 
there  is  a  considerable  difference. 


95 


CHAPTER  VI. 

EXPLOSION    IN    A    CLOSED   VESSEL. 

THE  value  of  any  inflammable  gas  for  the  production  of  power 
by  explosion,  can  be  determined  apart  altogether  from  theoretical 
considerations  by  direct  experiment.  It  is  evident  that  the  gas 
which  for  a  given  volume  causes  the  greatest  increase  in  pressure, 
will  give  the  greatest  power  for  every  cubic  foot  used,  provided 
that  the  pressure  does  not  fall  so  suddenly  that  it  is  gone  before  it 
can  be  utilised  by  the  piston. 

Two  qualities  will  be  possessed  by  the  best  explosive  mixture  : 
(i)  greatest  pressure  per  unit  volume  of  gas:  (2)  longest  time  of 
maximum  pressure  when  exposed  to  cooling. 

In  the  gas  engine  itself  the  conditions  are  so  complex  that  the 
problem  is  best  studied  in  the  first  instance  under  simplified  con- 
ditions. The  author  has  made  a  set  of  experiments  upon  many 
samples  of  coal  gas  mixed  with  air  in  varying  proportions,  to  find 
the  pressures  produced,  and  the  duration  of  those  pressures; 
igniting  mixtures  at  atmospheric  pressures  and  temperature,  and 
also  at  higher  temperature  and  initial  pressures.  He  has  made 
some  experiments  upon  pure  hydrogen  and  air  mixtures  in  the 
same  apparatus  for  comparison. 

The  experimental  apparatus  is  shown  at  fig.  19.  It  consists  of 
a  closed  cylindrical  vessel  7  inches  diameter  and  8|  inches  long, 
internal  measurement,  and  therefore  of  317  cubic  inches  capacity. 
It  is  truly  bored,  and  the  end  covers  turned  so  that  the  internal 
surface  is  similar  to  that  of  an  engine  cylinder  ;  the  covers  are 
bolted  strongly  so  as  to  withstand  high  pressures.  Upon  the 
upper  cover  is  placed  a  Richards  indicator,  in  which  the  reci- 
procating drum  has  been  replaced  by  a  revolving  one;  the  rate  of  re- 
volution is  adjusted  by  a  small  fan,  a  weight  and  gear  giving  the  power. 


96  The  Gas  Engine 

The  cylinder  is  filled  with  the  explosive  mixture  to  be  tester1; 
the  drum  is  set  revolving,  the  pencil  of  the  indicator  pressed 
gently  against  it,  and  the  electric  spark  is  passed  between  the 
points  placed  at  the  bottom  of  the  space.  The  drum  is  enamelled 
and  the  pencil  is  a  black-lead  one.  The  pressure  of  the  explo- 


+r-  REVOLVING  DRUM 


WEIGHT 


.  19.  —  Clerk  Explosion  Apparatus. 


tion  acts  upon  the  indicator  piston,  and  a  line  is  traced  upon  the 
drum,  which  shows  the  rise  and  fall  of  pressure.  The  rising  line 
traces  the  progress  of  the  explosion  ;  the  falling  line  the  progress 
of  the  loss  of  pressure  by  cooling.  The  rate  of  the  revolution  of 
the  drum  being  known,  the  interval  of  time  elapsing  between  any 
two  points  of  the  explosion  or  cooling  curve  is  also  known.  That 
is,  the  curve  shows  the  maximum  pressure  attained,  the  time  of 
attaining  it,  and  the  time  of  cooling.  Line  b  on  fig.  20  is  a  fac- 


Explosion  in  a  Closed  Vessel 


97 


simile  of  the  curve  produced  by  the  explosion  of  a  mixture  con- 
taining i  vol.  hydrogen  and  4  vols.  air.  Each  revolution  of  the 
drum  was  accomplished  in  0-33  s.-c.,  so  that  e.ich  tenth  of  a  revolu- 
tion takes  0*033  sec.  The  vertical 
divisions  give  time;  the  horizontal, 
pressures.  In  this  experiment  the 
maximum  pressure  produced  by  the 
explosion  is  68  Ibs.  per  square  inch 
above  atmosphere,  and  it  is  attained 
in  0-026  second.  Compared  with 
the  rate  of  increase  the  subsequent 
fall  is  very  slow.  The  rise  occurs  in 
0-026  second;  the  fall  to  atmo- 
sphere again  takes  1*5  second,  or 
nearly  sixty  times  the  other.  It  is 
in  fact  an  indicator  diagram  from  an 
explosion  where  the  volume  is  con- 
stant, the  motor  piston  being  absent, 
and  the  only  cause  of  loss  of  pres- 
sure is  cooling  by  the  enclosing 
walls.  The  exact  composition  of 
the  mixture,  its  uniform  admixture, 
the  temperature  and  pressure  before 
ignition,  are  all  accurately  known. 
After  studying  explosions  under 
these  known  conditions,  it  becomes 
easier  to  understand  what  occurs 
under  more  complex  conditions, 
where  the  moving  piston  makes  the 
cooling  surface  change,  and  where  the 
expansion  doing  work  also  requires 
consideration.  As  the  rapidity 
of  the  increase  of  pressure  measures 
the  explosiveness  of  a  mixture,  the 
time  occupied  from  the  commence- 
ment of  increase  to  maximum  pres- 
sure will  be  called  the  time  of  explosion. 
plete  when  maximum  pressure  is  attained. 


iput  'bs  jad  'sq[  ui 


The  explosion  is  corn- 
It  does  not  follow  from 
H 


98 


The  Gas  Engine 


UUOCJUOUU 


TJOU;  -bs 


Explosion  in  a  Closed  Vessel 


99 


this  that  the  combustion  is  complete;  that  is  another  matter.  The 
explosion  arises  from  the  rapid  spreading  of  the  flame  throughout  the 
whole  mass  of  the  mixture,  which  may  be  called  the  inflammation  of 
the  mixture.  More  or  less  rapid  inflammation  means  more  or 
less  explosive  effect,  but  not  complete  combustion.  The  complete 
burning  of  the  gases  present  does  not  occur  till  long  after  com- 
plete inflammation. 

The  terms  combustion,  explosion^  and  inflammation  will  be  used 
in  this  sense  alone  : 

Combustion,  burning  ;  complete  combustion,  the  complete 
burning  of  the  carbon  of  the  combustible  gas  to  carbonic  acid,  and 
the  hydrogen  to  water.  So  long  as  any  portion  of  the  combustible 
remains  uncombined  with  oxygen  the  combustion  is  incomplete. 

Complete  explosion,  the  attainment  of  maximum  pressure. 

Time  of  explosion;  the  time  elapsing  between  beginning  ot 
increase  and  maximum  pressure. 

Complete  inflammation,  the  complete  spreading  of  the  flame 
throughout  the  mass  of  the  mixture. 

Confusion  has  arisen  through  the  indifferent  use  of  these  terms, 
which  are  really  distinct  and  are  not  synonymous. 

With  mixtures  made  with  Glasgow  coal  gas  the  author  has 
obtained  the  following  maximum  pressures  and  times  of  explosion. 

EXPLOSION  IN  A  CLOSED  VESSEL.     (Clerk.) 
Mixtures  of  air  and  Glasgow  coal  gas. 

Temp,  before  explosion 18°  C. 

Pressure  before  explosion,  .....         .         .         .     atmospheric. 


Mixture                               Max'  Pres,s'  above  a?mos' 
in  pounds  per  sq.  in. 

Time  of  explosion 

Gas. 

Air. 

vol. 

13  vols. 

52 

O'28  sec. 

vol. 

ii  vols. 

63 

o'i8  sec. 

vol. 

9  vols. 

69 

o'i3  sec. 

vol. 

7  vols. 

89 

0*07  sec. 

vol. 

5  vols. 

96 

0x15  sec. 

The  highest  pressure  which  any  mixture  or  coal  gas  and  air 
is  capable  of  producing  without  compression  is  only  96  Ibs.  per 
sq.  in.  above  atmosphere  and  the  most  rapid  increase  is  not  more 
rapid  than  always  occurs  in  a  steam  cylinder  at  admission.  Many 


IOO 


The  Gas  Engine 


are  still  prejudiced  against  gas,  compared  with  steam,  because  of 
the  so-called  explosive  effect,  and  the  fear  that  gas  explosions 
may  occasion  pressures  quite  beyond  control,  like  solid  explosives. 
The  fear  is  quite  unfounded  ;  the  pressure  produced  by  the 
strongest  possible  mixture  of  coal  gas  and  air  is  strictly  limited 
by  the  pressure  before  ignition,and  can  always  be  accurately  known ; 
and  so  provided  for  by  a  proper  margin  of  safety  in  the  cylinders 
and  other  parts  subject  to  it. 

The  most  dilute  mixture  of  air  and  Glasgow  gas  which  can  be 
ignited  at  atmospheric  pressure  and  temperature  contains  T'4-  of 
its  volume  of  gas,  and  the  pressure  produced  is  52  Ibs.  above 
atmosphere.  The  time  of  explosion  is  0-28  second;  so  slow  is 
the  rise  that  it  cannot  with  justice  be  termed  an  explosion.  It  is 
too  slow  to  be  of  any  use  in  an  engine  running  at  any  reasonable 
speed  ;  the  stroke  would  be  almost  complete  before  the  pressure 
had  risen.  The  mixture  containing  6  volumes  of  gas  is  that 
with  just  enough  oxygen  to  burn  the  gas.  It  is  anomalous  that 
the  highest  pressure  is  given  with  excess  of  coal  gas.  The  rate  of 
ignition  also  is  greatest  with  that  mixture.  This  agrees  with  the 
results  obtained  by  Mallard  and  Le  Chatelier,  excess  of  hydrogen 
giving  the  highest  rate  of  inflammation. 

Similar  experiments  were  made  with  air  and  Oldham  coal  gas. 

EXPLOSION  IN  A  CLOSED  VESSEL.    (Clerk.) 
Mixtures  of  air  and  Oldham  coal  gas. 

Temp,  before  explosion 17°  C. 

Pressure  before  explosion atmospheric. 


Mixture 

Max.  press,  above  atmos. 
in  pounds  per  sq.  in. 

Time  of  explosion 

Gas. 

Air. 

I  vol. 

14  vols. 

40 

o'45  sec. 

vol. 

13  vols. 

5i'5 

o-3i  sec. 

vol. 

12  VOls. 

60 

0-24  sec. 

vol. 

ii  vols. 

61 

o'i7  sec. 

vol. 

9  vols. 

78 

o-o8  sec. 

vol. 

7  vols. 

87 

o-o6  sec. 

vol. 

6  vols. 

90 

ox>4  sec. 

vol. 

5  vols. 

9i 

0-055  sec- 

vol. 

4  vols. 

80 

o'i6  sec. 

The  highest  pressure  in  this  case  is  91  Ibs.  per  square  inch 


Explosion  in  a  Closed  Vessel 


101 


above  atmosphere,  but  the  most  rapid  explosion  is  0-04  second 
and  90  Ibs.  pressure,  a  little  less  pressure  than  is  given  by  Glasgow 
gas  but  a  slightly  more  rapid  ignition.  The  mixtures  are  evidently 
more  inflammable,  as  the  critical  mixture  is  ^  volume  of  gas 
instead  of  ^  as  with  Glasgow  gas.  Although  repeatedly  tried, 
a  mixture  of  i  volume  gas  15  volumes  air  failed  to  inflame  with 
the  spark. 

Hydrogen  and  air  mixtures  were  also  tested  as  follows  : 

EXPLOSION  IN  A  CLOSED  VESSEL.     (Clerk.) 
Mixtures  of  air  and  hydrogen. 

Temp,  before  explosion 16°  C. 

Pressure  before  explosion atmospheric. 


Mixture 

Max.  press,  above  atmos. 
in  pounds  per  sq.  in. 

Time  of  explosion 

Hyd. 

Air. 

i  vol. 

6  vols. 

41 

O'I5  sec. 

i  vol. 

4  vols. 

68 

o'O26  sec. 

2  VOls. 

5  vols. 

80 

o'oi  sec.         .  , 

The  inferiority  of  hydrogen  to  coal  gas,  volume  for  volume,  is 
very  evident ;  the  highest  pressure  is  only  80  Ibs.  above  atmosphere, 
and  the  mixture  requires  |  of  its  volume  of  hydrogen  to  give  it, 
while  coal  gas  gives  the  same  pressure  with  about  T\j  volume.  The 
hydrogen  mixture,  too,  ignites  so  rapidly  that  it  would  occasion 
shock  in  practice,  the  strongest  mixture  having  an  explosion  time 
of  one-hundredth  of  a  second.  With  gas  the  most  rapid  is  four- 
hundredths  of  a  second. 


THE  BEST  MIXTURE  FOR  USE  IN  NON-COMPRESSION  ENGINES. 

From  these  tables  can  be  ascertained  the  best  gas  and  the  best 
mixture  for  use  in  non-compression  engines  with  cylinders  kept 
cold.  Take  first  Glasgow  gas,  and  determine  which  mixture  gives 
the  best  result. 

(i)  Power  of  producing  pressure. 

Suppose  one  cubic  inch  of  Glasgow  coal  gas  to  be  used  in  each  of 
the  five  mixtures,  whose  maximum  pressures  and  times  of  explo- 
sion are  given  in  the  table  on  p.  99,  the  mixtures  would  measure 


IO2  The  Gas  Engine 

respectively  14,  12,  10,  8,  and  6  cubic  inches.  Let  them  be  placed 
in  cylinders  of  14,  12,  10,  8  and  6  square  inches  piston  area  ;  the 
piston  will  in  each  case  be  raised  one  inch  from  the  bottom  of  its 
cylinder.  If  the  pressures  upon  the  piston  were  the  same,  equal 
movements  of  piston  would  give  equal  power  ;  if  therefore  the 
mixtures  gave  equally  good  results  the  maximum  pressure  multiplied 
by  the  piston  area  will  in  all  cases  be  the  same. 

Multiplying  14,  12,  10,  8  and  6  by  their  corresponding  pres- 
sures 52,  63,  69,  89,  and  96  respectively,  the  products  are  728, 
756,  690,  712,  and  576.  These  numbers  are  the  pressures  in 
pounds  which  each  mixture  is  capable  of  producing  with  one 
cubic  inch  of  Glasgow  coal  gas,  cylinders  of  such  area  being  used 
that  the  depth  of  mixture  is  in  every  case  one  inch. 


Proportion  of  G'asgow  gas  in  mixture      Jj,    TV,    ^j,     \,      \. 
Pressure  produced  upon  p.stons  by  ,  ' 

one  cubic  inch          .         .  ' 

The  best  mixture  is  seen  at  a  glance  ;  it  is  that  containing 
one-twelfth  of  gas.  The  pressure  produced  by  one  cubic  inch  of 
gas  is  at  its  highest  value  756  pounds,  in  a  cylinder  of  12  inches 
piston  area,  and  containing  12  cubic  inches  of  mixture. 

In  modern  gas  engines  the  time  taken  by  the  piston  to  make 
the  working  part  of  its  stroke  is  generally  about  one-fifth  of  a 
second.  If  the  pressure  in  one  mixture  has  fallen  more,  proportion- 
ally in  that  time,  then  although  it  may  give  the  highest  maximum, 
it  may  lose  too  rapidly  to  give  the  highest  mean  pressure.  To  find 
this  cooling  effect,  find  the  pressure  to  which  each  mixture  falls 
at  the  end  of  0-2  second  after  maximum  pressure  ;  it  is  in  the 
different  cases  : 

Mixture  containing  gas       .  .    71T,      Jj,      •&,      \,       \. 

Time  after  beginning  explosion  (o~2  i          0         0 

-   0-48,  0-38,  0-33,  0-27,  0-25  sec. 
sec.  after  max.  pressure) 

Pressure  in  Ibs.  per  sq.  in  .         .         .     43,    48,    47,    55,    57. 

Press,  respectively  by  14,  12,  10,  8,  )    , 

*•  '       602,  576,  470,  440,  342. 


The  lower  row  expresses  the  relative  pressures  still  remaining 
after  allowing  each  explosion  to  cool  for  one-  fifth  of  a  second 
from  complete  explosion  ;  they  express  the  resistance  to  cooling 
possessed  by  the  mixtures.  It  is  evident  at  once  that  the 


Explosion  in  a  Closed  Vessel  103 

strongest  mixtures  cool  most  rapidly  ;  a  higher  temperature  being 
produced,  more  of  the  heat  of  the  explosion  is  lost  in  a  given  time. 

(2)  Power  of  producing  pressure  and  resisting  cooling. 

To  find  the  best  mixture  for  producing  pressure  and  resisting 
cooling,  those  numbers  are  to  be  added  to  the  corresponding  ones 
for  maximum  pressure  : 

Proportion  of  Glasgow  gas  in  mixture    Jy,    JJ(    7L,    i,     i. 

Pressure  produced  upon  pistons  by  i         0       ,    , 

[    728.  756.  690,  712,  576. 
one  cubic  men  gas          .         .         .  > 

Pressure  remaining  upon  pistons  o'2  i     , 

\    602,  576,  470,  440,  342. 
sec.  after  complete  explosion          . ) 

Mean  pressure 665,  666,  580,  576,  459. 

The  mean  of  the  two  sets  gives  numbers  expressing  the 
relative  values  of  the  mixture  for  producing  pressure,  and  at  the 
same  time  resisting  cooling.  The  two  weakest  mixtures  are  best 
in  both  respects,  the  low  result  given  by  the  strongest  mixture  is 
due  to  the  fact  that  excess  of  gas  is  present  and  it  remains  unburned, 
it  proves  how  easily  the  consumption  of  an  engine  may  be  increased 
by  even  a  slight  excess  of  gas  in  the  mixture. 

The  two  best  mixtures  ignite  too  slowly,  but  in  the  actual 
engine  that  is  easily  controlled,  as  will  be  explained  later. 
The  best  mixtures  are  i  vol.  gas  13  volumes  air,  and  i  vol. 
gas  ii  volumes  air.  With  more  gas  the  economy  will  rapidly 
diminish. 

The  experiments  with  Oldham  gas  treated  in  the  same  way 
give  the  following  results  : 

Proportion   of    Oldham    gas    in  \       ,       t        t        ,       ,       ,       j       ,        , 
mixture        .         .         .         .        \     ™    "'    «'    T2'    >"'     *'     Tl     *'      * 

Pressure  produced  upon  pistons  )    , 

\   noo,  721,  780,  732,  780,  606  6?o,  546,  400. 
by  one  cubic  inch  gas .         .        ' 

Pressure  remaining  upon  p:stons  \ 

o'2  sec.  after  complete  explo-   [•     3T>    4o,    4  •    44>    44.    47>    52<    5°-    4*>- 

sion  per  sq.  inch          .         .       ) 

Pressure  per  piston          .         .         .  465.  560,  546,  528,  440,  376,  364,  300,  230. 
Mean  pressure  upon  piston     .         .  532,  640,  663,  630,  610,  536,  497,  423,  315. 

Here,  too,  the  best  mixture  lies  between  one-twelfth  and  one- 
fourteenth  of  gas  ;  with  less  and  more  gas  the  result  becomes  worse 
and  worse.  Glasgow  and  Oldham  gases  seem  to  be  very  nearly- 
equal  in  value  per  cubic  foot  for  the  production  of  power,  as  the 


1O4  The  Gas  Engine 

pressure  produced  from  one  cubic  inch  in  the  best  mixture  of 
each  is  very  similar.  The  average  pressures  during  0-2  second 
from  complete  explosion  are  exceedingly  close,  Glasgow  gas 
mixture  containing  one-twelfth  gas  giving  666  Ibs.  pressure  per 
cubic  inch  of  gas,  and  Oldham  gas  for  the  same  mixture  and  the 
same  quantity  giving  630  Ibs.:  Glasgow  gas  one-fourteenth  mixture 
665  Ibs.  pressure,  Oldham  gas  640  Ibs.  The  hydrogen  experiments 
give  as  follows  : 

Proportion  of  hydrogen  gas  in  mixture  .        \,      i,      f. 
Pressure  produced  upon  pistons  bv  one  i       Q 

.....  r       2o7,  34^»  *oO. 

cubic  inch  hydrogen   .        .         .         .  > 
Pressure  remaining    upon   pistons  o'aj 

sec.  after  complete  explosion  per  sq.  j-       35,    39,    40. 

inch / 

Pressure  per  piston       .        .  245,  195,  140. 

Mean  pressure  upon  piston  .        .        .        266,  2^7,  210. 

The  best  mixture  with  i  cubic  inch  of  hydrogen  only  gives  a 
pressure  of  267  Ibs.  available  for  0-2  second,  so  that  its  capacity 
for  producing  power,  compared  with  Glasgow  and  Oldham  gas,  is 
as  267  is  to  665  and  640  respectively.  To  produce  equal  power 
with  Glasgow  gas  nearly  two-and  a-half  times  its  volume  of  hydrogen 
is  required.  The  idea  is  very  prevalent  among  inventors  that  if 
pure  hydrogen  and  air  could  he  used,  greater  power  and  economy 
would  be  obtained  ;  these  experiments  prove  the  fallacy  of  the 
notion.  Hydrogen  is  the  very  worst  gas  which  could  be  used  in 
the  cylinder  of  a  gas  engine,  it  is  useful  in  conferring  inflamma- 
bility upon  dilute  mixtures  of  other  gases,  but  when  present  in 
large  quantity  in  coal  gas  it  diminishes  its  value  per  cubic  foot  for 
power. 

PRESSURES  PRODUCED  IF  NO  Loss  OR  SUPPRESSION 
OF  HEAT  EXISTED. 

From  the  fact  already  mentioned  in  the  last  chapter,  that  the 
theoretical  temperatures  of  combustion  are  never  attained  in 
reality,  it  will  naturally  be  expected  that  the  pressures  produced 
by  explosions  in  closed  vessels  will  also  fall  short  of  theory. 
This  is  found  to  be  the  case.  It  has  been  observed  by  every 
experimenter  upon  the  subject,  beginning  with  Him  in  1861, 
who  determined  the  pressures  produced  by  the  explosion  of  coal 


Explosion  in  a  Closed  Vessel  105 

gas  and  air,  and  hydrogen  and  air.  He  used  two  explosion  vessels 
of  3  and  36  litres  capacity  ;  they  were  copper  cylinders  with  dia- 
meters equal  to  their  length.  He  used  a  Bourdon  spring  mano- 
meter to  register  the  pressure.  He  states  that  : 

(1)  With  10  per  cent,  hydrogen  introduced  the  results  were  : 
according  to  experiment,  3*25  atmospheres  ;  according  to  calcu- 
lation, 5 '8  atmospheres. 

(2)  With  20  per  cent,  of  hydrogen,  the  results  were  :  according 
to  experiment,  7  atmospheres,  which  is  very  much  below  the  cal- 
culation. 

(3)  With  10  per  cent,  of  lighting  gas  introduced  the  results 
were  :  according  to  experiment,  5  atmospheres,  i.e.  much  more 
than  with  the  introduction  of  an  equal  volume  of  pure  hydrogen. 

He  notices  especially  the  low  pressure  produced  by  hydrogen 
as  compared  with  lighting  gases,  but  observes  truly  that  this  should 
not  excite  surprise— although  the  heat  value  of  hydrogen  is  great, 
yet  it  is  so  when  compared  with  equal  weights  of  other  substances — 
and  that  coal  gas  being  four  or  five  times  as  heavy  as  hydrogen, 
quantity  is  balanced  against  quality  ;  therefore  volume  for  volume 
it  gives  out  more  heat. 

He  considers  that  there  is  no  difficulty  in  explaining  the  very 
considerable  difference  found  between  calculation  and  experiment, 
as  the  metal  sides  are  at  so  low  a  temperature  compared  with  the 
explosion,  that  the  heat  is  rapidly  conducted  away,  and  the 
attainment  of  the  highest  temperature  is  impossible.  Bunsen,  in 
his  experiments,  observed  the  same  difference,  and  so  later  did 
Mallard  and  Le  Chatelier.  The  author's  experiments  fully 
confirm  the  accuracy  of  those  observers.  In  no  case,  whether 
with  weak  or  strong  mixtures  of  coal  gas  and  air,  or  hydrogen 
and  air,  is  the  pressure  produced  which  should  follow  the  com- 
plete evolution  of  heat. 

Thus,  with  hydrogen  mixtures  (Clerk's  experiments)  : 

Per  sq.  in. 

i  vol.  H  6  vols.  air  gives  by  experiment       .         .       41  Ibs.  above  atmosphere. 
The  calculated  pressure  is    .         .         .         .88-3 

1  vol.  H  4  vols.  air  experiment  gives  ...       68 

Calculated  pressure  is  .         .         .         .         .124 

2  vols.  H  5  vols.  air  experiment  gives          .         .       80 

Calculated  pressure  is .         .         .         .        .     176 


io6 


The  Gas  Engine 


Without  exception  the  actual  pressure  falls  far  short  of  the 
calculated  pressure  ;  in  some  manner  the  heat  is  suppressed  or 
lost.  That  the  difference  cannot  altogether  be  accounted  for  by 
loss  of  heat  is  easily  proved ;  the  fall  of  pressure  is  so  slow  from 
the  maximum  that  it  is  impossible  that  any  considerable  proportion 
of  heat  can  be  lost  in  the  short  time  of  explosion.  If  so  large  a 
proportion  were  lost  on  the  rising  curve,  it  could  not  fail  to  show 
upon  the  falling  curve  ;  it  would  fall  in  fact  as  quickly  as  it  rose. 
Again,  the  increase  of  pressure  would  be  less  in  a  small  than  in  a 
large  vessel,  as  the  small  vessel  exposes  the  larger  surface  pro- 
portionally to  the  gas  present.  It  is  found  that  this  is  not  so. 
Bunsen  used  a  vessel  of  a  few  cubic  centimetres  capacity,  and  got 
with  carbonic  oxide  and  oxygen  true  explosive  mixture  10*2  atmo- 
spheres maximum  pressure  ;  Berthelot  with  a  vessel  4000  cb.  c. 
capacity  got  io'i  atmospheres  ;  with  hydrogen  true  explosive 
mixture  Bunsen  9^5  atmospheres,  Berthelot,  9^9  atmospheres.  All 
the  difference,  therefore,  cannot  be  accounted  for  by  loss  before 
complete  explosion. 

Mixtures  of  air  and  coal  gas  give  similar  results. 

The  following  are  the  observed  and  calculated  pressures  for 
Oldham  coal  gas.  (Clerk's  experiments.} 

Per  sq.  in. 
i  vol.  gas  14  vols.  air,  experiment  gives 

Calculated  pressure  is  .        . 
i  vol.  gas  13  vols.  air,  experiment  gives 

Calculated  pressure  is  . 
i  vol.  gas  12  vols.  air,  experiment  gives 

Calculated  pressure  is  . 
I  vol.  gas  1 1  vols.  air,  experiment  gives 

Calculated  pressure  is  .         .         .         ,         .112 
I  vol.  gas  9  vols.  air,  experiment  gives         .         .       78 

Calculated  pressure  is  .         .         .         .         .134 
t  vol.  gas  7  vols.  air,  experiment  gives         .         .       87 

Calculated  pressure  is  .         .         .         .         .     168 
I  vol.  gas  6  vols.  air,  experiment  gives         .         .       90 

Calculated  pressure  is  .         .         .         .         .     192 

The  results  with  Glasgow  gas  are  so  similar  that  it  is  unneces- 
sary to  give  a  table  ;  in  no  case  does  the  maximum  pressure 
account  for  much  more  than  one-half  of  the  total  heat  pre- 
sent. As  all  of  the  deficit  cannot  have  disappeared  previous 
to  complete  explosion,  it  follows  that  the  gases  are  still  burning 
on  the  falling  curve,  that  is,  the  falling  curve  does  net  truly 


40  Ibs.  above  atmosphere 
893 


96 

60 

103 

61 


Explosion  in  a  Closed  Vessel  107 

represent  the  rate  of  cooling  of  air  heated  to  the  maximum  tem- 
perature, because  heat  is  being  continually  added  by  the  continued 
combustion  of  the  mixture.  This  will  be  fully  proved  by  a  study 
of  the  curves. 

It  may,  however,  be  taken  as  completely  proved  by  the 
complete  accord  of  all  physicists  who  have  experimented  on  the 
subject,  that  for  some  reason  nearly  one-half  of  the  heat  present 
as  inflammable  gas  in  any  explosive  mixture,  true  or  dilute,  is 
kept  back  and  prevented  from  causing  the  increase  of  pressure  to 
be  expected  from  it.  Although  differences  of  opinion  exist  on  the 
cause,  all  are  agreed  on  the  fact ;  they  also  agree  in  considering 
that  inflammation  is  complete  when  the  highest  pressure  is 
attained. 

TEMPERATURES  OF  EXPLOSION. 

With  a  mass  of  any  perfect  gas  confined  in  a  closed  vessel  the 
absolute  temperatures  and  pressures  are  always  proportional ;  double 
temperature  means  double  pressure.  Temperatures  T,  /  (absolute), 

T  P 

pressures  corresponding  P,/ ;  then  -=  -  (Charles's  law).  If  ex- 
plosive mixtures  behaved  as  perfect  gases,  the  pressure  before 
explosion  and  temperature  being  known,  the  pressure  of  ex- 
plosion at  once  gives  the  corresponding  temperature.  It  has 
been  shown  at  page  82  that  explosive  mixtures  do  not  fulfil 
this  condition,  but  change  in  volume  from  chemical  causes 
quite  apart  from  physical  ones.  It  follows,  therefore,  that  these 
changes  must  be  kno\vn  before  the  temperature  of  the  explosion 
can  be  calculated  from  the  pressure.  In  the  cases  of  hydrogen 
and  carbonic  oxide  true  explosive  mixtures  with  oxygen,  a 
contraction  of  volume  is  the  result  of  combination.  It  comes  to 
the  same  thing  as  if  a  portion  of  the  perfect  gas  in  the  closed 
vessel  was  lost  during  heating  ;  the  temperature  then  could  not 
be  known  at  the  higher  pressure  unless  the  volume  lost  is  also 
known. 

Suppose  one-third  of  the  volume  to  disappear,  upon  cooling 
to  the  original  temperature,  the  pressure  would  be  reduced  to 
two-thirds  of  the  original  pressure,  and  this  fraction  of  the 
original  pressure  must  be  taken  as  /1  =  io.  As  both  steam  and 


io8  The  Gas  Engine 

carbonic  acid  at  temperatures  high  enough  to  make  them  per- 
fectly gaseous  occupy  two-thirds  of  the  volume  of  their  free 
constituents,  it  follows  that  />,  must  be  taken  as  §  /,  wherever 
the  temperatures  are  such  that  combination  is  complete.  But 
here  another  difficulty  occurs.  Bunsen  found  that  hydrogen 
and  oxygen  in  true  explosive  mixtures  gave  an  explosion  pressure 
of  9'5  atmospheres.  The  calculated  pressure  for  complete  combus- 
tion, and  allowing  for  chemical  contraction  is  21  -3  atmospheres.  It 
is  evident  enough  that  complete  combustion  has  not  occurred,  but 
it  is  difficult  to  say  what  fraction  remains  uncombined.  Yet 
unless  the  fraction  in  combination  be  known  the  contraction  cannot 
be  known,  and  therefore  the  temperature  corresponding  to  the 
pressure  cannot  be  known. 

Berthelot  has  pointed  out  that  in  a  case  of  this  kind  the  true 
temperature  cannot  be  calculated,  but  it  may  be  shown  to  lie 
between  two  extreme  assumptions,  both  of  which  are  erroneous. 

(1)  Temperature  calculated  on  assumption  of  no  contraction. 

(2)  Temperature  calculated   on  assumption  of  the  complete 
contraction. 

Let  the  two  temperatures  be  (i)  x1  and  (2)  T. 

T1  T 

2vols.  H,  i  vol.  O,  explosion  pressure)        2449" C.         3809° C. 

(absolute)  9*9  atmospheres          .         .  I 
2  vols.  CO,  i  vol.  O,  explosion  p-essure)         2612°  C          4i-io°C 

(absolute)  io'8  atmospheres        .         .  ) 

The  lower  temperature  could  only  be  true  if  no  combination 
whatever  had  occurred,  which  is  impossible,  as  then  no  heat  at  all 
could  be  evolved;  the  higher  temperature  could  only  be  true  if  com- 
plete combination,  and  therefore  complete  contraction,  occurred. 
The  truth  is  somewhere  between  these  numbers. 

When  the  explosive  mixture  is  dilute,  the  limits  of  possible 
error  are  narrower,  because  the  possible  proportion  of  contraction 
is  less  ;  with  hydrogen  and  air  mixture  in  proportion  for  complete 
combination,  2  volumes  of  hydrogen  require  5  volumes  of  air. 
The  greatest  possible  contraction  of  the  7  volumes  is  therefore  i 
volume.  If  all  the  hydrogen  burned  to  steam,  the  7  volumes 
contract  to  6  volumes.  With  more  dilute  mixtures  the  pro- 
portion diminishes. 

With   a   mixture  containing  \  of  its   volume    hydrogen,    10 


Explosion  in  a  Closed  Vessel 


109 


volumes  can  only  suffer  contraction  to  9  volumes.    With  \  volume 
hydrogen,  14  volumes  can  contract  to  13  volumes. 

The  limits  of  maximum  temperatures  for  those  mixtures  are  as 
follows  (Clerk)  : 


i  vol.  H,  6  vols.  air,  explosion  pressure  | 
(absolute),  55  7  Ibs.  per  sq.  in.  .         .  ! 

1  vol.  H,  4  vols.  air,  explosion  pressure  | 
(absolute),  827  Ibs.  per  sq.  in. .         .  i 

2  vols.  H,  5  vols.  air,  explosion  pressure  \ 

(absolute),  947  Ibs.  per  sq.  in.  .         ,  I 


T, 

826°  C. 

1358° C. 
1615°  C. 


T 
909°  C 

1539° C. 
1929°  C. 


The  possible  error  is  here  much  less  than  with  true  explosive 
mixtures  ;  coal  gas  is  of  such  a  composition  that  some  of  its 
constituents  expand  upon  decomposition  previous  to  burning,  and 
so  to  some  extent  balance  the  contraction  produced  by  the 
burning  of  the  others.  The  possible  error  is  therefore  still  further 
reduced.  The  composition  of  Manchester  coal  gas  as  determined 
by  Bunsen  and  Roscoe  is  as  below.  The  oxygen  required  for  the 
complete  combustion  of  each  constituent  is  also  given,  and  the 
volumes  of  products  formed. 

ANALYSIS  OF  MANCHESTER  COAL  GAS.     (Bunsen  and  Roscoe.) 


Amount  required 
for  complete 
combustion 

Products 

Hydrogen,  H       . 
Marsh  gas,  CH4  . 
Carbonic  oxide,  CO 
Ethylene,  CoH4  . 
Tetrylene,  C4H3  . 
Sulphuretted  hydrogen,  H2S 
Nitrogen,  N 
Carbonic  acid,  CO2     . 

vols. 

45-58 

34  '9 
6-64 
4-08 
2-38 
0*29 

2  '46 
3-67 

vols.  O 
2279 
69-8 
3'32 

12  '24 
I4-28 
0'43 

vols. 
45-58,  HoO 
1047,    CO2  &  H-jO 
6-64,  CO2 
16-32,  CO2&H,O 
19-04,  CO,  &  HoO 

0-58,  H2b&sb2 

2-46 

3-67 

Total    .... 

lOO'OO 

122-86  O 

198-99,  CO2H2O&SO2 

When  burned  in  oxygen  100  volumes  of  this  sample  of  gas 
require  122-86  volumes  of  oxygen,  total  mixture  222-86  volumes; 
the  products  of  the  combustion  measure  198 '99  volumes.  Calcula- 
ting to  percentage,  100  volumes  of  the  mixture  will  contract  to  89-4 


I  10 


The  Gas  Engine 


volumes  of  the  products.  As  100  volumes  ot  the  mixture  will 
contain  55*1  volumes  of  oxygen,  it  follows  that  if  air  be  used,  four 
times  that  volume  of  nitrogen  will  be  associated  with  it,  that  is, 
55-1  x  4  =  220*4.  The  strongest  possible  explosive  mixture  ot 
this  coalgas  with  air  containing  100  volumes  of  the  true  explosive 
mixture  will  be  3 20 '4  volumes,  and  it  will  contract  upon  complete 
combustion  to  309-8  volumes. 

One  volume  of  this  gas  requires  6-14  volumes  air  for  complete 
combustion,  and  100  volumes  of  the  mixture  contract  to  96^6 
volumes  of  products  and  diluent.  A  contraction  of  3*4  per  cent. 
Dilution  still  further  diminishes  the  change ;  thus  a  mixture-,  i 
volume  gas  13-28  volumes  air,  will  have  only  half  that  contraction, 
or  i -7  per  cent. 

From  these  figures  it  is  evident  that  the  limits  of  possible 
error  in  calculating  temperature  from  pressure  of  explosion  does 
not  exceed,  in  the  worst  case,  with  coal  gas  and  air  3-4  per  cent., 
and  in  weaker  mixtures  half  that  number.  The  fact  that  the  whole 
heat  is  not  evolved  at  the  explosion  pressure,  and  that  therefore 
the  whole  contraction  does  not  occur  then,  further  reduces  the 
error.  It  is  then  nearly  correct  to  calculate  temperature  from 
pressure  without  deduction  for  contraction.  This  has  been  done 
for  Glasgow  gas  and  for  the  Oldham  gas  experiments  by  the 
author. 

EXPLOSION  IN  A  CLOSED  VESSEL.     (Clerk.) 
Mixtures  of  air  and  Glasgow  coal  gas. 

Temp,  before  explosion 18°  C. 

Pressure  before  explosion       ......     atmos.  147  Ibs. 


Mixture 

Max.  press,  above  atmos. 
in  pounds  per  sq.  in. 

Temp,  of  explosion 
calculated  from 
observed  pressure 

Gas. 

Air. 

I  vol. 

13  vols. 

52 

1047°  C. 

I  vol. 

ii  vols. 

63 

I265°C. 

I  vol. 

9  vols. 

69 

1384°  c. 

i        I  Vol. 

7  vols. 

89 

1780°  C. 

i  vol. 

j  vols. 

96 

1918   C. 

Explosion  in  a  Closed  Vessel 


III 


Mixtures  of  air  and  Oldham  coal  gas. 
Temp,  before  explosion        ..... 


C. 


Mixture 

Max.  press,  above 
atmos.  in  pounds 

Temp,  of  explosion 
calculated  from 

Theoretical  temp, 
of  explosion  if  all 

per  sq.  in. 

observed  pressure 

heat  were  evolved 

Gas. 

Air. 

vol. 

14  vols. 

40 

806°  C. 

1786°  C. 

vol. 

13  vols. 

51  '5 

1033°  C. 

1912  ;  C. 

vol. 

12  VOlS. 

60 

1202°  C. 

2058  C. 

vol. 

ii  vols. 

61 

1220°  C. 

2228-  C. 

vol. 

9  vols. 

78 

1557°  C. 

2670°  C. 

vol. 

7  vols. 

87 

1/33°  C. 

3334°  C. 

vol. 

6  vols. 

90 

17^2°  C. 

3808   C. 

vol. 

5  vols. 

91 

1812   C. 

vol. 

4  vols. 

80 

i595°C. 

Those  temperatures  calculated  from  maximum  pressure, 
although  not  quite  true  are  very  nearly  so,  whatever  be  the  theory 
adopted  to  explain  the  great  deficit  of  pressure.  It  does  not  follow, 
however,  that  they  are  the  highest  temperatures  existing  at  the 
moment  of  explosion  ;  they  are  merely  averages.  The  existence  of 
such  an  intensely  heated  mass  of  gas  in  a  cold  cylinder  causes 
intense  currents,  so  that  the  portion  in  close  contact  with  the 
cold  walls  will  be  colder  than  that  existing  at  the  centre.  There 
will  be  a  hot  nucleus  of  considerably  higher  temperature  than 
that  outside,  but  whatever  that  temperature  may  be,  the  increase 
of  pressure  gives  a  true  average.  It  may  be  taken,  then,  that 
coal  gas  mixtures  with  air  give  upon  explosion  temperatures 
ranging  from  800°  C  to  nearly  2000°  C.,  depending  on  the  dilution 
of  the  mixture.  The  more  dilute  the  mixture  the  lower  the 
maximum  temperature  ;  increase  of  gas  increases  maximum  tempe- 
rature at  the  same  time  as  it  increases  inflammability. 

The  author  has  made  explosion  experiments  in  the  same 
vessel  with  mixtures  previously  compressed,  and  finds  that  the 
pressures  produced  with  any  given  mixture  are  proportional  to 
the  pressure  before  ignition,  that  is,  with  a  mixture  of  constant 
composition,  double  the  pressure  before  explosion,  keeping  tempe- 
rature constant  at  18°  C.,  doubles  the  pressure  of  explosion.  The 
experiments  are  laborious,  and  they  are  not  yet  complete  for  pub- 
lication, but  the  general  principles  already  developed  are  true  for 
compressed  mixtures  also. 


112  The  Gas  Engine 

EFFICIENCY  OF  GAS  IN  EXPLOSIVE  MIXTURES. 

Rankine  defines  available  heat  as  follows  : 

'  The  available  heat  of  combustion  of  one  pound  of  a  given 
sort  of  fuel  is  that  part  of  the  total  heat  of  combustion  which  is 
communicated  to  the  body  to  heat  which  the  fuel  is  burned  ; 
and  the  efficiency  of  a  given  furnace,  for  a  given  sort  of  fuel,  is  the 
proportion  which  the  available  heat  bears  to  the  total  heat.' 

The  gas  engine  contains  furnace  and  motor  cylinder  in  one  ; 
nevertheless  the  efficiency  of  the  working  fluid  is  quite  as  distinct 
from  the  furnace  efficiency  as  in  the  steam  engine.  Rankine's  de- 
finition is  quite  true  for  the  gas  engine. 

The  fuel  being  gas,  the  working  fluid  consists  of  air  and  its  fuel 
and  their  combinations  ;  the  available  heat  is  that  part  of  the 
heat  of  combustion  which  serves  to  raise  the  temperature  of  the 
working  fluid  ;  the  part  which  flows  into  it  to  make  up  for  loss  to 
the  cold  cylinder  walls  cannot  be  considered  available.  To  be 
truly  available  it  must  either  increase  temperature,  or  keep  it 
from  falling  by  expansion.  The  heat  flowing  through  the 
cylinder  walls  is  a  furnace  loss,  incident  to  the  explosion  method 
of  heating. 

The  experiments  upon  explosion  in  a  closed  vessel  provide 
data  for  determining  the  furnace  efficiency  as  distinguished  from 
that  of  the  working  fluid.  The  proportion  of  heat  flowing  from 
an  explosion  to  the  walls  in  unit  time  will  depend  upon  the 
surface  of  the  walls  for  any  given  volume.  The  smaller  the 
cooling  surface  in  proportion  to  volume  of  heated  gases,  the 
slower  will  be  the  rate  of  cooling.  Therefore  to  be  applicable  to 
any  engine,  the  explosion  vessel  in  which  the  experiments  are 
made  should  have  the  same  capacity  and  surface  as  the  explosion 
space  of  the  engine. 

The  author's  experiments  are  therefore  only  strictly  applicable 
to  engines  with  cylinders  similar  to  his  explosion  vessel.  Within 
certain  limits,  however,  the  error  introduced  by  applying  them  to 
other  engines  is  inconsiderable. 

Assuming  the  stroke  of  a  gas  engine  (after  explosion)  to  take 
o-2  second,  this  may  be  taken  as  the  time  during  which  the 
pressure  of  explosion  must  last  if  it  is  to  be  utilised  by  the 


Explosion  in  a  Closed  Vessel  1 1 3 

engine.  In  a  closed  vessel  the  pressure  falls  considerably  in  0-2 
second,  the  average  pressure  may  be  taken  as  nearly  indicating 
the  available  pressure  during  that  time.  The  heat  necessary  to 
produce  that  pressure  is  the  available  heat  ;  and  its  proportion  to 
the  total  heat  which  the  gas  present  in  the  mixture  can  evolve  is 
the  efficiency  of  the  gas  in  that  explosive  mixture. 

With  Oldhamgas  the  best  mixture  is  (table,  p.  103)  i  volume 
gas  12  volumes  air  ;  the  average  pressure  during  the  first  fifth  of 
a  second  is  51  Ibs.  per  square  inch  above  atmosphere.  If  all 
the  heat  present  heated  the  air,  the  pressure  should  be  103  Ibs. 
effective,  so  that  the  efficiency  of  the  heating  method  is  ^jj  = 
0-49. 

The  strongest  mixture  which  still  contains  oxygen  in  excess 
is  i  volume  gas  7  volumes  air,  the  average  available  pressure  is 
67  Ibs.  per  square  inch  (all  heat  evolved  would  give  168  Ibs.),  the 
efficiency  is  y6^  =  0-40  nearly. 

Calculated  in  this  way  the  efficiency  values  for  Oldham  gas 
mixtures  are  : 

Prop,  of  Oldham  gas  in  mixture  .       TV-     TV'      TS-      tV-     T\>'      \<       \- 
Heating  efficiency        .         .         .     0^40,  o'48,  o'5o,  o  43,  o'46,  0^40,  o'37. 

The  furnace  efficiency  plainly  diminishes  with  increased 
richness  of  the  mixture  in  gas. 

TIME  OF  EXPLOSION  IN  CLOSED  VESSELS. 

The  rates  of  the  propagation  of  flame  in  explosive  mixtures 
given  in  tables,  pages  86  and  87,  are  true  only  where  the 
inflamed  portion  is  free  to  expand  without  projecting  itself  into 
the  unignited  portion.  They  are  the  rates  proper  for  constant 
pressure. 

Where  the  volume  is  constant,  in  a  closed  vessel,  the  part  first 
inflamed  instantly  expands  and  so  projects  the  flame  surface  into 
the  mass,  compressing  what  remains  into  smaller  space. 

To  the  rate  of  inflammation  at  constant  pressure  are  added 
the  projection  of  the  flame  into  the  mass  by  its  expansion 
and  also  the  increased  rate  of  propagation  in  the  unignited 
portion  by  the  heating  due  to  its  compression  by  portion  first 
inflamed. 

i 


1 14  The  Gas  Engine 

It  follows  that  the  rate  continually  increases,  as  the  inflamma- 
tion proceeds  until  it  fills  the  vessel. 

This  is  evident  from  all  the  explosion  curves.  The  pressure 
rises  slowly  at  first,  then  with  ever  increasing  rate  till  ihe  explosion 
is  complete  ;  thus  the  explosion  curve  for  hydrogen  mixture  with 

(2  \ 

-   H   I,  shows  an  increase  of  17  pounds  in  the  first  0-005 

second,  the  maximum  pressure  of  So  pounds  being  attained  in  the 
next  o-oo5  second.  With  the  weaker  mixtures  the  same  thing 
occurs,  rise  of  pressure,  slow  at  first,  then  more  rapid,  and  in 
some  cases  becoming  slow  again  before  maximum  pressure.  The 
time  taken  to  ret  maximum  pressure  varies  much  with  the  circum- 
stances attending  the  beginning  of  the  ignition.  If  a  considerable 
mass  be  ignited  at  once,  by  a  long  and  powerful  spark,  or  by  a 
large  flame,  the  ignition  of  the  weakest  mixture  may  be  made 
almost  indefinitely  rapid.  Something  very  like  Berthelot's  explo- 
sive wave  may  result.  This  is  due  to  the  great  mechanical 
disturbance  caused  by  the  rapid  expansion  of  the  portion  first 
ignited  ;  the  smaller  that  portion  is  the  more  gently  does  the 
flame  spread.  A  small  separate  chamber  connected  with  the 
main  vessel,  if  filled  with  explosive  mixture  and  ignited,  will 
project  a  rush  of  flame  into  the  main  vessel  and  cause  almost 
instantaneous  ignition.  The  shape  of  the  vessel,  too,  has  a  great 
effect  upon  the  rate.  Where  it  is  cylindrical  and  large  in  diameter 
proportional  to  its  axial  length,  ignition  is  extremely  rapid,  the 
flame  is  confined  at  starting,  and  is  rapidly  deflected  by  the 
cylinder  ends,  and  so  shoots  through  the  whole  mass. 

By  so  arranging  the  explosion  space  of  a  gas  engine  that  some 
mechanical  disturbance  is  permitted,  it  is  easy  to  get  any  required 
rate  of  ignition  even  with  the  weakest  mixtures. 

The  maximum  pressure  is  not  increased  by  rapid  ignition. 

Starting  the  ignition  from  a  small  spark,  the  time  taken  to 
ignite  increases  with  the  volume  of  the  vessel. 

Berthelot  has  experimented  upon  this  point  with  explosion 
vessels  of  three  capacities,  300  cubic  centimetres,  1500  cubic 
centimetres,  and  4000  cubic  centimetres.  He  finds  time  of 
explosion  (he  also  takes  maximum  pressure  to  indicate  complete 


Explosion  in  a  Closed  -Vessel  1.15. 

explosion)  of  mixture  2  vols.  H,  i  vol.  O,  and  2  vols.  N,  in  300 
cubic  centimetre  vessel,  0-0026  second ;  and  in  4000  cubic 
centimetre  vessel,  0*0068  second. 

With  mixture  of  carbonic  oxide  and  oxygen,  2  vols.  CO,  i  vol. 
O,  smaller  vessel,  0-0128  second;  larger  vessel,  0-0155  second. 
Mixtures  with  air  were  much  slower.  The  conclusion  then  is 
obvious,  that  in  large  engines  the  time  of  explosion  will  be  longer 
than  in  small  ones. 


u6  The  Gas  Engine 


CHAPTER  VII. 

THE   GAS    ENGINES   OK   THE   DIFFERENT    TYPES    IN    PRACTICE. 

HAVING  now  studied  the  theoretic  efficiency  of  the  different 
kinds  of  engine  and  the  mechanism  of  the  heating  method— that 
is  the  properties  of  gaseous  explosions — the  way  is  clear  for 
the  study  of  the  results  obtained  from  the  engines  in  practice. 

It  is  quite  evident  that  no  practicable  engine  can  give  an 
efficiency  at  all  approaching  theory  from  the  use  of  gaseous 
explosions  ;  the  temperatures  and  therefore  pressures  produced 
fall  far  short  of  that  due  to  the  complete  evolution  of  the  heat 
present  in  the  mixture  as  combustible  gas.  All  the  heat  of 
the  gas  does  not  go  to  increase  the  temperature  of  the  working 
fluid  ;  a  large  proportion  of  it  is  rendered  latent  in  some  way 
when  the  maximum  temperature  is  attained. 

The  appearance  of  the  diagrams  from  the  explosion  of  mixtures 
commonly  used  in  gas  engines,  shows  at  first  a  very  rapid  increase 
of  pressure  and  temperature,  which  terminates  abruptly  and  is 
immediately  succeeded  by  a  fall  which  is  relatively  a  slow  one. 

It  was  formerly  supposed  that  the  completion  of  the  explosion 
was  coincident  with  the  completion  of  the  combustion,  and  there- 
fore of  the  evolution  of  heat.  This,  however,  was  shown  by 
Bunsen  and  those  who  have  followed  him,  to  be  untrue  ;  although 
the  temperature  ceases  to  rise,  and  fall  sets  in,  the  gas  present 
has  in  few  explosions  been  more  than  half-burned  at  the  moment 
of  maximum  temperature.  The  causes  which  suppress  the  heat  of 
the  explosion  and  prevent  it  from  being  evolved  at  once  are  com- 
plex and  have  occasioned  different  explanations  which  will  be  fully 
discussed  in  a  subsequent  chapter.  Meantime,  it  is  sufficient 
to  recognise  the  fact  and  to  understand  its  bearing  upon  the 
economy  of  gas  engines. 


Gas  Engines  of  Different  Types  in  Practice         1 17 

It  is  a  phenomenon  common  to  all  gas  engines  which  have 
ever  been  constructed,  whether  using  compression  previous  to  ig- 
nition or  not.  The  heat  so  suppressed  appears  when  cooling  sets  in, 
and  consequently  explosive  mixtures  cool  more  slowly  in  appear- 
ance than  would  a  mass  of  air  heated  to  similar  temperatures  and 
exposed  to  similarly  cold  enclosing  walls. 

In  many  gas  engines  the  indicator  diagrams  are  apparently 
almost  perfect,  that  is,  the  lines  of  falling  temperatures  are 
almost  true  adiabatics.  So  far  as  the  diagram  }ields  informa- 
tion, the  gases  in  expanding  are  losing  no  heat  whatever  to  the 
cylinder,  but  the  temperature  is  falling  apparently  only  by  work  done 
upon  the  piston.  This  supposition  is  known  to  be  untrue,  because 
the  gases  are  at  a  temperature  often  as  high  as  the  hottest  of 
blast  furnaces,  and  the  walls  enclosing  are  at  most  at  the  boiling 
point  of  water.  It  is  the  suppressed  heat  which  is  being  evolved 
during  fall  of  temperature  which  sustains  the  temperature  and 
makes  the  diagram  appear  as  if  no  loss  or  but  little  was  going  on. 
An  actual  engine  therefore  may  give  a  diagram  which  is  the  exact 
theoretical  one,  and  yet  the  efficiency  of  the  engine  be  much 
below  theory.  The  authors  experiments  upon  explosive  mixtures 
were  undertaken  to  get  the  data  necessary  for  the  interpretation  of 
the  diagram,  and  the  rising  and  falling  curves,  showing  times  of 
rise  and  fall  of  pressure,  give  the  efficiency  of  coal  gas  in  the 
different  mixtures,  apart  altogether  from  theoretic  considerations. 
Whatever  the  opinions  held  regarding  the  cause  or  causes  of  the 
suppression  of  heat,  the  experiments  with  carefully  proportioned 
explosive  mixtures,  at  known  temperatures  and  pressures,  deter- 
mine absolutely  the  capability  of  gas  for  producing  pressure  and 
for  sustaining  it  under  cooling. 

As  the  efficiency  may  be  very  different  from  that  shown  by 
the  indicator,  it  is  advisable  to  distinguish  between  the  real  and 
apparent,  efficiency.  Call  the  one  apparent  indicated  efficiency, 
and  the  other  actual  indicated  efficiency. 

The  apparent  indicated  efficiency,  when  multiplied  by  the  effi- 
ciency  of  the  gas  in  the  particular  mixture  used,  will  givethe  actual 
indicated  efficiency.  For  instance,  if  the  diagram  gave  the  efficiency 
cf  an  engine  as  0-29  and  the  efficiency  of  the  mixture  was  0-48,  then 
the  actual  indicated  efficiency  is  0-29  x  0-48=0-1 1.  That  is,  only 


n8  The  Gas  Engine 

0-48  of  the  gas  present  when  the  diagram  is  taken  really  acts 
in  producing  elevation  of  temperature;  the  remaining  0-52  is  sup- 
pressed and  keeps  up  the  temperature,  which  would  otherwise  fall 
by  cooling.  The  diagram  alone  can  never  tell  accurately  the 
losses  which  are  taking  place  unless  the  heat  is  all  evolved  at  once 
and  appears  in  temperature;  then,  but  not  till  then,  will  the  lines 
traced  by  the  indicator  tell  the  loss  of  heat.  Some  previous 
writers  have  misinterpreted  their  indicator  diagrams  through 
neglect  of  this  fact. 

Some  others,  notably  Dr.  Slaby,  of  Berlin,  have  assumed  that 
the  phenomenon  of  retarded  combustion  is  produced  by  invention 
and  occurs  only  in  the  Otto  engine.  This  is  a  mistake.  All  engines 
using  explosion  necessarily  exhibit  it  ;  in  fact,  as  it  is  an  accom- 
paniment of  all  explosions,  it  is  impossible  to  make  an  engine  in 
which  it  is  avoided. 

In  the  following  examination  of  the  performances  of  the 
various  engines  in  practice  the  importance  of  the  phenomenon  will 
appear. 

Type  i. — The  most  important  engines  of  this  type  which  have 
yet  been  in  public  use  are  those  of  Lenoir,  Hugon,  and  Bisschoff. 
Many  others  have  been  made  and  sold  in  some  numbers,  but  as 
these  three  present  fully  all  the  peculiarities  of  the  type,  it  would 
only  waste  time  to  describe  the  varied  mechanical  details  consti- 
tuting the  sole  novel  points  in  the  others. 

LENOIR  ENGINE. 

The  Lenoir  engine  as  made  differs  considerably  from  that 
described  in  his  specifications.  As  a  rule  of  almost  general  appli- 
cation, specifications  are  untrustworthy  as  accurate  descriptions  of 
working  machines  ;  the  author  has  been  careful  to  describe  no 
engine  which  he  has  not  examined. 

Fig.  22  is  a  section  of  the  cylinder  of  a  half-horse  power 
Lenoir  engine.  The  engine  in  the  Patent  Office  Museum,  South 
Kensington,  is  well  made  and  in  external  appearance  closely  re- 
sembles an  ordinary  high-pressure  steam  engine. 

The  cylinder  is  5^  inches  diameter,  and  the  stroke  is  Scinches. 
Its  cylinder  is  provided  with  two  valves  ;  both  are  slides,  working 


Gas  Engines  of  Different  Types  in  Practice         1 19 

between  the  cylinder  faces  and  covers,  which  ar°  held  down  to  the 
slides  by  adjusting  screws.  One  valve  controls  the  discharge 
of  the  products  of  combustion,  the  other,  the  admission  and 
mixing  of  the  inflammable  gas  and  air.  The  ignition  is  effected 
by  the  electric  spark.  The  working  cycle  of  the  engine  is  as 
follows  : 

When  the  piston  is  at  the  end  of  its  stroke,  the  gas  and  air  ad- 
mission valve  is  open  ;  the  main  port  in  ic  opens  to  the  atmosphere, 


MIXING 
PLATE 


FIG.  22. — Lenoir  Engine  Cylinder  (sectional  plan). 

while  a  smaller  port  leads  from  the  main  port  to  the  gas  supply. 
The  forward  movement  of  the  piston  draws  into  the  cylinder  the 
air  and  the  gas,  which  mix  as  they  enter  the  main  valve  port  and  the 
engine  admission  port.  At  about  half-stroke  the  supply  of  mixed 
gases  is  cut  off,  so  that  the  cylinder  is  completely  closed  off  from 
the  atmosphere  and  from  the  gas  supply  ;  an  electric  spark  now 
passed  into  the  explosive  mixture  from  a  battery  and  induction  coils 
causes  explosion  and  the  pressure  rapidly  rises.  The  piston  is 


1 20  The  Gas  Engine 

thereby  pushed  on  its  stroke  during  the  portion  remaining  to 
be  completed  ;  at  the  end  of  the  stroke  the  pressure  has  fallen 
by  expansion  doing  work  and  by  the  cooling  action  of  the  cylinder 
walls,  to  nearly  atmosphere  again  ;  the  exhaust  valve  opens  and 
during  the  return  stroke  the  products  of  combustion  are  expelled 
preparatory  to  taking  in  a  fresh  charge  upon  the  next  working  stroke. 
The  same  operation  is  repeated  upon  the  other  side  of  the  piston 
so  that  the  engine  is  double-acting  of  a  kind.  It  cannot  be  con- 
sidered as  truly  double-acting,  like  the  steam  engine,  as  the  driving 
pressure  is  not  acting  during  the  whole  forward  stroke,  but  only 
during  that  portion  of  it  which  is  not  taken  up  in  sucking  in 
the  explosive  charge.  The  fly  wheel,  because  of  this,  is  much 
larger  than  in  a  steam  engine  of  corresponding  dimensions, 
and  the  power  is  also  much  less.  The  valves  are  both  actuated  by 
eccentrics  upon  the  crank  shaft.  Each  slide  requires  a  separate 
eccentric  because  the  exhaust  during  the  whole  stroke  and  the 
admission  during  only  half-stroke  could  not  be  managed  by  the 
single  to  and  fro  movement.  To  get  the  best  result  it  is  evident 
that  the  least  possible  power  should  be  expended  in  introducing 
the  charge  ;  therefore,  large  inlet  air  ports  are  required,  all  the 
larger  because  an  eccentric  cannot  be  made,  alone,  to  give  a 
sudden  cut-off.  To  prevent  throttling  as  the  ports  approach 
the  closing  points,  the  total  opening  must  be  considerable.  The 
eccentric  is  so  set  that  the  port  is  cpen  slightly  before  the  crank 
has  crossed  the  centre,  so  that  it  may  be  well  open  when  the 
charge  begins  to  enter.  In  fact  it  has  some  lead  like  a  steam 
slide,  and  for  the  same  purpose.  The  exhaust  valve  is  set 
precisely  as  in  the  steam  engine,  and  is  of  similar  construction, 
except  that  it  is  not  enclosed  in  a  case,  but  it  is  held  against  the 
cylinder  face  by  a  cover  and  screws.  Fig.  22  is  a  sectional  plan 
of  the  cylinder,  showing  the  valves  and  ports  ;  fig.  23  is  a  trans- 
verse vertical  section  of  the  cylinder,  showing  the  valves  and  valve 
.  covers  with  gas,  air  and  exhaust  ports.  The  arrows  indicate  the 
direction  of  the  gas  and  air  flow,  while  mixing  and  entering  the 
cylinder,  also  the  exhaust  path.  As  the  air  port  opens  to  the 
cylinder  slightly  before  the  piston  has  completed  its  stroke,  and 
a  slight  pressure  may  yet  remain  in  the  cylinder,  the  gas  port 
does  not  open  till  a  little  later.  The  gas  and  air  do  not  open 


Gas  Engines  of  Different  Types  in  Practice         121 

quite  simultaneously,  although  nearly  so  ;  neither  do  they  close 
quite  together.  There  is  one  gas  admission  port  in  the  slide 
leading  into  the  main  port ;  in  the  cover  there  are  two  ports 
between  which  the  slide  port  passes,  taking  gas  from  either,  as 
is  required,  for  the  end  of  the  cylinder  which  is  receiving  the 
charge.  The  main  valve  port  opens  on  the  upper  side  to  the  air, 
and  is  covered  by  a  perforated  plate  and  a  light  metal  case 
furnished  with  a  throttle  valve  ;  the  brass  plate  perforated  is 
carried  downwards  and  covers  the  gas  port,  so  that  the  gas 
entering  from  tbe  supply  pipe  is  not  permitted  to  flow  at  once  into 


Gas  Inlet 
FlG.  23.  —  Lenoir  Engine  Cylinder  (transverse  section). 

the  main  port,  but  must  first  pass  up  through  the  perforations  and 
mix  with  the  air  which  is  going  down  through  those  adjoining. 
The  mixing  arrangement  is  somewhat  imperfect,  and  is  exceed- 
ingly sensitive  to  change  of  speed  in  the  engine.  The  throttle-valve 
is  intended  to  increase  or  diminish  the  supply  of  air  ;  by  closing 
it  slightly,  the  suction  upon  the  gas  port  is  increased,  and  so 
the  proportion  of  the  mixture  altered.  By  opening  it,  the  air  has 
freer  access  to  the  cylinder,  the  pressure  is  not  reduced  so  much, 
and  therefore  the  gas  is  diminished.  The  mixing,  however,  is  too 
irregular  ;  as  the  gas  streams  are  not  projected  separately  into  the 


122  The  Gas  Engine 

incoming  air  stream,  the  cas  flows  too  much  in  mass  into  the  air 
in  mass.  The  igniting  points  were  invariably  placed  at  the  upper 
part  of  the  cylinder  in  the  cylinder  covers.  The  cylinder  and  covers 
are  waterjacketed,  the  water  is  kept  continuously  flowing  through, 
so  that  the  temperature  may  not  become  so  high  as  to  injure  the 
cylinder.  This  is  a  most  necessary  precaution  in  any  gas  engine 
of  even  moderate  power  ;  the  effect  of  neglect  in  a  Lenoir  engine 
is  very  soon  observed  in  complete  cutting  up  of  the  cylinder  ;  it 
speedily  becomes  red-hot  if  allowed  to  run  without  water.  In- 
deed, even  with  an  adequate  water  supply,  the  larger  engines  gave 
great  trouble  ;  although  the  cylinder  could  be  kept  cool  the  piston 
could  not.  It  was  proportioned  too  much  on  steam  engine  lines, 
and  when  working  at  full  power  the  incessant  explosions  upon  both 
sides  caused  so  rapid  a  flow  of  heat  into  it  that  the  small  surface 
exposed  to  the  water  jacket  by  the  circumference  was  insufficient 
to  carry  away  the  heat  absorbed  by  the  whole  piston  area.  The 
pistons  often  became  red-hot. 

The  exhaust  slide  also  had  rather  hard  work,  and  required 
delicate  adjustment,  as  the  exhaust  gases  were  very  hot,  often 
800°  C. ;  the  expansion  of  the  slide  was  therefore  considerable, 
and  in  order  to  be  pressure  tight  when  hot,  the  adjusting  screws 
had  to  be  kept  rather  easy  when  cold.  The  engine  when 
starting,  therefore,  always  leaked  a  little  at  the  exhaust  valve. 
The  same  thing  happened  with  the  admission  slide,  but  to  a  lesser 
degree. 

Notwithstanding  the  large  area  of  the  admission  port  and  the 
lead  given  to  the  admission  valve,  the  closing  motion  was  too 
slow  to  prevent  throttling  ;  accordingly  the  pressure  fell  some- 
what below  atmosphere,  while  the  valve  was  cutting  off  preparatory 
to  explosion.  After  cutting  off  a  slight  delay  occurred  between  the 
passing  of  the  spark  and  the  commencement  of  the  explosion  ; 
the  explosion  itself  took  some  time  to  complete  ;  it  was  by  no 
means  instantaneous  ;  the  diagram  produced  was  consequently 
imperfect.  In  addition  to  all  this,  the  piston  being  so  hot,  heated 
the  charge  while  it  was  entering,  and  so  occasioned  further 
loss. 

The  lubricating  arrangements  also  were  primitive.  The 
steam  engine  requiring  but  little  care  in  lubricating,  the  gas 


Gas  Engines  of  Different  Types  in  Practice         123 

engine  was  not  supposed  to  require  more ;  and  the  ordinary  lubri- 
cating cock  was  deemed  sufficient.  All  these  sources  of  loss, 
inevitable  in  a  first  attempt,  made  the  engine  comparatively  in- 
efficient. Notwithstanding  all  its  defects,  the  Lenoir  engine  at  the 
time  of  its  production  was  the  best  the  world  had  yet  seen,  and  in 
careful  hands  it  did  good  work  and  created  a  widespread  interest. 
The  engine  in  South  Kensington  Museum,  under  the  skilful  care  of 
Mr.  S.  Ford,  worked  for  many  years  supplying  all  the  power  required 
for  the  repair  department  of  the  Patent  Office  Museum.  It  runs  with 
perfect  smoothness,  nothing  whatever  in  its  action  would  enable 
one  standing  beside  it  to  imagine  for  a  moment  that  the  motive 
power  was  explosive.  The  popular  notion  of  an  explosion  is  always 
associated  with  the  idea  of  a  great  noise.  This,  of  course,  physicists 
have  always  known  to  be  a  fallacy,  as  no  explosion  makes  noise 
unless  it  has  access  to  the  atmosphere.  An  explosion  in  a  closed 
vessel  makes  no  sound  unless  the  vessel  bursts.  In  a  gas  engine 
it  is  only  necessary  to  see  that  the  explosion  is  not  too  rapid,  but 
that  time  is  allowed  for  the  slack  of  the  connecting  rod  and  crank 
connections  to  take  up.  The  explosions  used  by  Lenoir  were 
seldom  more  rapid  in  rise  of  pressure  than  is  common  with  all 
steam  engines.  A  i -horse  Lenoir  engine  inspected  lately  by  the 
author  at  Petworth  House,  Petworth,  had  been  at  work  for  the 
past  twenty  years  pumping  water  for  the  town  and  is  still  at  work. 
It  works  with  smoothness  and  is  altogether  more  silent  in  its  action 
than  most  modern  gas  engines.  The  author  finds  that  many  Lenoir 
engines  are  still  at  work  after  twenty  years'  continuous  use,  notably 
two  i -horse  power  engines  at  the  Brewery  of  Messrs.  Trueman, 
Hanbury  and  Buxton,  London,  and  one  i -horse  power  at  the 
establishment  of  Messrs.  Day,  Son  and  Hewitt,  Dorset  Street, 
London,  all  doing  hard  work  with  great  regularity. 

Diagrams  and  Gas  consumption. — Prof.  Tresca  of  Paris  has  made 
experiments  with  a  ^-horse  Lenoir  engine,  and  found  that  it  con- 
sumed 95  cb.  ft.  of  Paris  gasper  indicated  horse  power  per  hour.  The 
diagrams  from  so  small  an  engine  hardly  do  justice  to  the  method, 
and  as  it  is  desirable  to  compare  the  engine  with  modern  engines 
using  similar  volumes  of  charge  the  author  has  taken  a  diagram  from 
a  paper  by  Mr.  Slade,  published  in  the  Journal  of  the  Franklin 
Institute,  Philadelphia.  The  engine  had  a  cylinder  of  eight  inches 


I24 


The  Gas  Engine 


diameter  and  sixteen  stroke.  The  explosion  space  corresponds 
closely  to  that  of  the  author's  experimental  explosion  vessel. 

The  diagram,  fig.  24,  at  once  shows  the  truth  of  the  preceding 
discussion  of  the  action  of  the  engine. 

AB  is  the  atmospheric  line,  traced  upon  the  indicator  card 
by  the  pencil  before  opening  the  indicator  cock  to  communicate 
with  the  interior  of  the  cylinder  ;  it  is  the  neutral  position  of  the 
indicator  piston  while  the  pressure  on  both  sides  of  it  is  at  atmo- 
sphere; any  pressure  from  within  the  cylinder  pushes  up  the  piston 
and  therefore  the  indicator  pencil.  Pressure  above  atmosphere 
is  registered  by  lines  above  that  line,  pressure  below  atmosphere 


Diagram  at  50  revolutions,  cylinder  £J  inches  diameter,  i6\  inches  s  roke. 
FIG.  24.  —  Lenoir  Engine  Diagram. 

is  registered  by  lines  below  that  line.  The  card  shows  three  dis- 
tinct tracings,  each  corresponding  to  one  stroke  of  the  engine  : 
admission  of  the  charge,  explosion,  expansion  and  return  expel- 
ling the  products  of  the  combustion.  If  the  cycle  is  carried 
out  in  a  mechanically  perfect  manner  the  admission  of  the  charge 
should  be  accomplished  without  loss  by  throttling.  This  is  not  so. 
Fiom  the  point  a  to  the  point  b  the  valve  is  open  enough  to  give 
free  access  to  the  cylinder,  and  accordingly  the  pressure  within  the 
cylinder  is  not  appreciably  lower  than  that  without  ;  but  here  the 
valve  begins  to  contract  its  opening  at  the  very  moment  that 
the  piston  is  moving  most  rapidly,  the  pressure  falls  and  is  a  couple 
of  pounds  per  square  inch  below  atmosphere  when  it  closes. 
When  closed,  the  spark  does  not  at  once  take  effect,  so  that  the 
pressure  has  become  n  Ibs.  per  sq.  in.  total  before  the  igni- 


Gas  Engines  of  Different  Types  in  Practice         125 

tion  begins  to  cause  a  rise.  Then  the  ignition  itself  takes  some 
time  to  be  completed,  here  about  ^  second  ;  the  piston  has, 
therefore,  moved  through  a  further  one-and-a-half-tenth  of  its 
stroke  and  the  heat  given  by  the  explosion  is  not  added  at  strictly 
constant  volume,  as  required  by  theory.  Apart  altogether  from 
loss  of  heat  to  the  cylinder  walls,  this  diagram  is  mechanically  im- 
perfect. The  valve  arrangements  should  be  such  that  no  loss  is 
incurred  in  charging  and  that  the  explosion  follows  so  rapidly  that 
the  pressure  in  the  cylinder  has  no  time  to  fall  by  expansion, 
after  closing  the  admission  ;  the  explosion,  indeed,  should  at  once 
follow  the  cut-off.  In  the  best  of  the  three  lines  the  pressure  has 

2035°  C.  absolute 


••  623    C.  abs. 


FIG.  25.  -  Lenoir  Engine  Diagram. 

fallen  to  nearly  u  Ibs.  total,  and  the  maximum  pressure  of  the 
explosion  is  48  Ibs.  per  square  inch  total.  The  average  of  the 
three  lines  gives  a  pressure  divided  over  the  whole  stroke  of  only 
8'3  Ibs.  per  square  inch,  which,  assuming  the  diagram  from  the 
other  end  of  the  cylinder  to  give  similar  results,  gives  a  total  ot 
2  indicated  horse  power  at  50  revolutions  per  minute.  This  is 
an  exceedingly  poor  result  for  so  large  an  engine.  The  apparent 
indicated  efficiency  is  much  below  that  of  a  theoretical  diagram 
using  the  same  expansion.  Fig.  25  shows  in  dotted  lines  a 
diagram  which  will  have  the  same  efficiency  as  the  actual  diagram 
(best  of  the  three  lines).  If  the  temperature  of  the  entering 
charge  has  been  raised  to  100°  C.,  as  stated  by  Mr.  Slade,  then 
the  point  c  upon  the  diagram  corresponds  to  that  tempera- 
ture; the  point  d  will  correspond  to  a  temperature  of  2035°  C. 
absolute,  as  the  volume  has  increased  from  0-4  to  0-5  and  the 


1  26  The  Gas  Engine 

pressure  from  u  Ibs.  to  14-7  Ibs.  per  square  inch  total  at  the  point 
e  .  The  area  of  the  part  of  the  explosion  curve  def  may  be  taken 
as  equal  to  the  part  of  the  diagram  cfg  which  is  resistance  due  to 
the  valve  action  ;  the  work  done  upon  the  piston  by  the  one  part 
balances  the  loss  by  the  other  :  both  portions  may  therefore  be 
neglected,  the  dotted  lines  representing  the  apparent  diagram 
efficiency. 

The  temperatures  for  calculating  maximum  possible  efficiency 
are  as  follows  —  they  are  also  marked  upon  the  diagram 

T  2035°  absolute. 

Tl  1534° 

t  623° 

/>....     1246° 

Calculating  E  from  formula  (17)  p.  57 

1-408  (/'-/) 


_ 


T-/ 


__  t   _  (1534-  1246)   .-  1-408  (1246  -  623) 

2°35-623 
=  0-175. 

The  apparent  indicated  efficiency  for  the  best  of  the  three 
lines  is  0-175.  ^  ^  were  constantly  repeated,  the  actual  indicated 
efficiency  may  be  obtained  by  multiplying  by  the  efficiency  of  the 
gas  in  the  mixture  used  to  get  the  explosion.  The  numbers  got 
from  explosion  in  a  closed  vessel  do  not  quite  represent  the  con- 
ditions of  loss  in  a  cylinder  with  a  moving  piston.  In  the  first 
case  the  loss  ot  pressure  and  temperature  is  due  solely  to  the 
cooling  effect  of  the  vessel's  walls;  in  the  second  the  moving 
piston  reduces  pressure  and  temperature  by  expansion,  and  at  the 
same  time  increases  the  surface  exposed.  The  increased  surface, 
however,  will  not  increase  the  rate  of  cooling,  as  the  volume  is  at 
the  same  time  increased  in  a  greater  proportion.  It  has  been 
already  shown  that  cooling  of  a  heated  mass  of  gas  is  indepen- 
dent of  the  pressure,  and  depends  on  the  ratio  of  surface  to 
volume. 

In  the  engine  the  volume  of  the  hot  gases  becomes  doubled  by 


Gas  Engines  of  Different  Types  in  Practice         1 27 

expansion,  but  the  surface  exposed  does  not  double;  the  cylinder 
surface  increases  with  the  volume,  but  the  piston  area  and  cylinder- 
cover  area  remain  the  same,  so  that  the  proportion  of  surface  to 
volume  diminishes  instead  of  increasing.  The  heat  lost  to  the 
cylinder  and  piston  and  cover  in  the  engine  will  therefore  be  no 
greater  than  that  lost  to  the  enclosing  wall  of  the  experimental 
explosion  vessel  in  a  similar  time.  It  will  indeed  be  somewhat 
less,  as  in  the  time  taken  doing  work  the  temperature  will  fall  by 
heat  disappearing  as  work.  With  the  closed  vessel  the  fall  is  due 
solely  to  cooling,  so  that  the  average  temperature  during  the  time  of 
exposure  is  higher.  More  work  is  urgently  required  by  careful 
physicists  to  get  accurate  data.  At  present  the  approximation  to 
the  efficiency  of  the  gas  in  different  mixtures  by  closed  vessel 
experiments  is  the  best  that  can  be  had;  it  cannot  be  greatly  in 
error.  The  efficiencies  obtained  from  the  indicator  diagram  and 
the  author's  experiments  will  be  lower  than  the  truth,  the  more  so 
the  greater  the  expansion.  With  engines  as  at  present  constructed 
the  difference  is  but  small. 

The  mixture  required  to  give  a  temperature  of  2035°  C.  absolute 
is,  for  Oldham  gas,  i  gas  6  air,  and  the  average  pressure  during 
o-3  sec.  from  complete  explosion  is  63  Ibs.  per  square  inch  above 
atmosphere,  nearly.  The  time  taken  to  expand  in  the  engine 
after  explosion  is  0-3  sec.  ;  the  pressure  which  should  be  produced 
by  the  explosion  of  this  mixture,  if  all  the  heat  of  the  gas  went 
to  heat  the  air  and  products,  192  Ibs.  per  square  inch  above  atmo- 
sphere. That  is,  the  difference  between  192  and  63  has  gone  in 
heat  suppressed  at  the  moment  of  complete  explosion  and  heat 
lost  while  exposed  to  the  influence  of  the  vessel  walls  during  the 
same  period  as  the  effective  stroke  of  the  engine. 

The  efficiency  of  the  gas  in  the  mixture  is  therefore 

6? 

J  =  0-33  nearly, 
192 

that  is,  only  one-third  is  really  effective  in  raising  temperature. 
The  actual  indicated  efficiency  will,  therefore,  be  only  one -third 
of  the  apparent.  Three  times  the  amount  of  heat  accounted  for 
by  the  diagram  is  required  to  make  the  gases  used  in  the  explosion 
show  the  temperatures  and  curve  of  the  diagram. 


128  The  Gas  Engine 

Apparent  indicated  efficiency  x  efficiency  of  gas  •=  actual 
indicated  efficiency  : 

0-175  x  °'33  =  °'°58 

The  actual  indicated  efficiency  of  the  engine  is  0-058  or  5-8 
per  cent,  if  this  diagram  be  constantly  repeated;  but  as  it  is  the 
best  of  the  three  lines  it  requires  correction.  Taking  the  worst  of  the 
three  diagrams,  fig.  24  shows  the  temperature  as  follows  :  T,  2035° 
absolute;  T1,  1697°  absolute  ;  /,  797°  ;  /',  1243°  absolute. 

The  apparent  indicated  efficiency  is  E  =  0-126. 

The  actual  indicated  efficiency  is  0-126  x  0-33  =  0-0495  or 
4 -95  per  cent,  of  the  total  heat  given  to  the  engine. 

Tresca  calculates  the  heat  transformed  into  work  by  the  Lenoir 
tested  by  him  as  4  per  cent. 

The  mean  of  the  best  and  worst  of  these  diagrams  is 

T8  +  4'Q"> 

* — Y^  =  5'37i 

which  is  higher  than  the  result  obtained  by  this  distinguished  physi- 
cist ;  but  the  difference  is  sufficiently  accounted  for  by  the  differ- 
ence in  the  dimensions  of  the  engines.  Tresca's  was  only  half- 
horse,  Slade's  was  two  horse. 

The  Lenoir  engine  used  mixtures  ranging  in  composition  from 
i  gas  and  6  vols.  air  to  i  vol.  gas  and  12  vols.  air,  depending  upon 
the  amount  of  work  upon  the  engine  ;  when  there  was  little  work 
the  governor  was  arranged  to  throttle  the  gas  and  so  diminish  the 
proportion  present.  This  was  a  bad  plan,  as  will  be  explained  in  the 
chapter  upon  governing.  But  the  effect  was  to  make  the  engine 
use  all  grades  of  ignitable  mixtures  from  the  strongest  to  the 
weakest.  Apart,  however,  from  all  intentional  arrangements  for 
governing,  these  engines  tended  to  govern  themselves.  An  increase 
of  speed  always  causes  the  proportion  of  gas  in  the  mixture  to 
diminish,  because  the  resistance  of  the  small  gas  port  to  flow 
increases  more  rapidly  than  the  larger  air  port.  It  follows  that  if 
the  ports  are  proportioned  to  pass  certain  volumes  at  a  low  rate  of 
speed,  at  a  higher  rate  the  proportion  is  disturbed,  the  smaller 
port  giving  a  greater  proportional  resistance.  The  effect  is  seen 
in  all  the  diagrams,  the  ignitions  become  later  and  later  as  the 
mixture  diminishes  in  inflammability,  and  after  attaining  a  certain 


Gas  Engines  of  Different  Types  in  Practice         1 29 


dilution,  ignition  ceases  altogether,  or  becomes  too  slow  to  be  of 
any  practical  use.  In  the  Lenoir  type  of  engine  too  slow  ignition 
is  an  unmixed  evil,  as  the  theory  of  the  engine  requires  rapid  igni- 
tion. In  it  the  loss  of  efficiency  due  to  valve  and  igniting  arrange- 
ments is  considerable.  The  electric  ignition  is  very  delicate  and 
troublesome.  To  overcome  the  defects  of  the  Lenoir,  Hugon 
introduced  his  engine,  which  in  some  respects  was  a  considerable 
advance. 

HUGON  ENGINE. 

The  Hugon  engine,  like  the  Lenoir,  exploded  the  charge  drawn 
into  the  cylinder  by  the  piston  at  atmospheric  pressure  :  in  it, 
however,  greater  expansion  and  more  dilute  mixtures  were  used. 


Hugon  Engine  Cylinder. 


FIG.  27. 


Fig.  26  is  a  sectional  plan  showing  valves,  passages  and  the 
cylinder  and  piston.  Fig.  27  is  a  transverse  vertical  section 
through  the  cylinder  at  the  line  ab.  The  admission  of  the  charge 
and  the  expelling  of  the  exhaust  are  accomplished  through  the 
same  passage,  so  that  the  cylinder  has  only  two  ports,  as  in  the 
steam  engine ;  two  valves  are  used,  one  working  outside  the  other. 
The  inner  valve  has  five  ports,  two  for  admitting  the  charge,  one 
for  exhausting,  and  two  for  carrying  the  igniting  flame.  The 
ignition  by  flame  was  first  accomplished  in  a  workable  manner  by 
Hugon,  although  it  had  been  described  in  several  patents  long 
before  his  time. 

The  ports  marked  i  i  in  the  inner  slide  A  are  admission,  the 


1 30  The  Gas  Engine 

ports  marked  2  2  in  the  inner  slide  are  igniting,  ports ;  the  port  3 
is  the  exhaust  passage,  alternately  communicating  with  each  end 
of  the  cylinder  by  the  long  ports  4  4  to  the  exhaust  port  5,  pre- 
cisely as  in  a  steam  engine.  The  action  of  the  admission  ports 
is  somewhat  novel.  The  object  is  to  secure  a  rapid  opening  and 
cut-off,  bringing  the  igniting  flame  on  immediately  after  closing 
the  cylinder.  The  valve  is  actuated  from  a  cam.  When  the 
piston  is  at  the  end  of  its  stroke  and  is  moving  forward,  the 
valve  A  is  moving  in  the  same  direction,  the  port  3  is  allowing 
the  exhaust  gases  to  escape  from  the  other  side  of  the  piston,  the 
port  i  is  open  to  the  cylinder  and  is  communicating  through  the 
port  6  or  6,  in  the  outer  slide  B,  with  the  air  and  also  with 
the  gas  supply.  When  the  piston  has  taken  in  sufficient  charge, 
the  cam  moves  the  slide  A  suddenly  forward,  so  causing  the  port  i 
to  close  on  the  outer  side  but  not  on  the  inner ;  the  igniting 
port  comes  on  and  the  flame  burning  in  it  inflames  the  mixture, 
filling  the  engine  port,  from  whence  it  spreads  into  the  cylinder 
itself.  As  the  inner  valve  cuts  off  when  moving  in  the  same 
direction  as  it  does  when  opening,  it  is  evident  that  it  must  cross 
back  again,  to  be  in  the  position  required  to  commence  opening 
at  the  correct  time.  While  crossing,  unless  the  communication 
with  the  atmosphere  and  gas  supply  is  stopped  in  some  other  way, 
it  will  open  at  the  wrong  time ;  to  prevent  this,  the  outer  valve  B 
is  provided.  It  is  actuated  from  a  pin  projecting  from  the  main 
valve  A  ;  this  pin  7  works  in  the  slot  8,  and  while  the  main  valve 
is  moving  forward  after  cutting  off,  the  pin  strikes  the  end  of  the 
slot  and  carries  the  outer  valve  with  it,  causing  it  to  close  the  port 
in  the  cover  which  it  commands.  A  small  plate  and  spring  give 
friction  enough  to  keep  the  valve  in  position  till  it  is  moved  in 
the  other  direction.  When  tlia  main  valve  returns,  although  its 
ports  open  on  the  engine  ports,  the  outer  ends  are  blinded  by  the 
outside  valve  which  is  not  again  opened  till  the  main  valve  has 
closed.  By  this  ingenious  contrivance,  a  rapid  admission  and 
cut-off  are  secured  with  one  cam  and  the  main  and  auxiliary 
slides.  The  engine  from  which  these  details  are  taken  is  in  South 
Kensington  Museum  and  is  rated  at  i-horse  power.  The  valves 
are  arranged  to  cut  off  at  about  one-third  stroke. 

The  cylinder  is  8r3^  diameter  and  10  in.  stroke.    The  clearance 


Gas  Engines  of  Different  Types  in  Practice         131 

spaces  due  to  the  long  ports  4  4,  the  valve  ports  open  to  the 
cylinder  at  the  moment  of  explosion,  and  the  space  into  which  the 
piston  does  not  enter,  make  up  in  all  a  proportion  of  products  of 
combustion  equivalent  to  nearly  thirty  per  cent,  of  the  entire  charge. 
The  effect  of  this  is  to  cause  a  considerable  difference  between 
the  nature  of  the  mixture  in  the  port  and  that  in  the  cylinder 
itself,  the  port  mixture  being  much  more  inflammable  than  that 
in  the  cylinder.  As  a  consequence  the  ignition  is  more  rapid 
with  weak  mixtures  than  in  the  Lenoir.  The  gas  is  supplied  to 
the  air  port  in  regulated  amount  by  means  of  a  bellows  pump 
worked  from  an  eccentric  on  the  crank  shaft ;  it  mixes  with  the  air 
in  passing  through  the  valves  and  port ;  the  products  of  combustion 
are  therefore  completely  expelled  from  the  port,  and  nothing  but 
pure  mixture  left  to  be  inflamed  by  the  igniting  arrangement 
The  gas  for  the  internal  igniting  flame  is  supplied  also  from  a 
bellows  pump  under  slight  pressure.  This  flame  is  extinguished 
by  each  explosion,  and  is  relighted  when  the  port  cpens  again  to 
the  air  by  a  constant  external  flame.  The  action  of  the  exhaust 
port  in  the  main  slide  is  so  evident  as  to  require  no  other 
explanation  than  that  afforded  by  the  drawing. 

The  engine  works  very  smoothly,  and  is  a  great  improvement 
upon  Lenoir  in  certainty  of  action  ;  all  the  trouble  with  the 
battery  and  coil  is  very  simply  avoided.  To  prevent  overheat- 
ing of  the  piston,  water  is  injected  by  means  of  a  tap ;  it  is 
adjusted  so  that  each  suck  of  the  engine  drawing  in  mixture  also 
takes  in  enough  water  to  keep  the  piston  at  a  reasonable  tempe- 
rature. In  this  the  engine  was  successful ;  it  was  capable  of 
harder  and  more  continuous  work  than  the  Lenoir,  and  was  in 
every  way  more  certain  in  its  action  even  with  a  considerable 
variation  in  the  composition  of  the  explosive  mixture  used.  The 
only  parts  which  gave  trouble  were  the  bellows  pumps  controlling 
the  gas  supply  to  cylinder  and  igniting  port  ;  these  were  made  of 
rubber,  and  deteriorating  after  some  use  gave  trouble  by  leaking 
and  occasional  bursting.  In  some  of  the  engines  in  use  they 
were  replaced  by  metal  pumps  and  a  mixing  valve.  With  these 
additions  the  engine  in  the  Patent  Orifice  Museum  ran  for  many 
years. 

Diagrams  and  Gas  Consumption. — According  to  Professor 

K  2 


132 


TJie  Gas  Engine 


Tresca,  the  gas  consumed  by  a  Hugon  engine  of  2 -horse  power 
was  85  cubic  feet  per  indicated  horse  per  hour. 

Fig.  28  is  a  diagram  taken  from 
a    ^-horse    engine    by   the    author. 
The    engine    was    indicating    078 
horse  power,    the   average  pressure 
^      being  3-9  Ibs.,   and  the   maximum 
^      25  Ibs.  per  sq.  in.     The  card  shows 
considerable  delay  in  explosion  after 
c      cut-off,    notwithstanding    the    rapid 

movement  of  the  igniting  slide. 

• 

Cu 

«  BISCHOFF  ENGINE. 

E 

^  The   consumption   of  the  non- 

compression  type  of  engine  is  too 
^  high  to  permit  of  its  use  in  any  but 
§  the  very  smallest  machines  ;  accord- 
g  ingly  the  Lenoir  and  Hugon  engines 
|£  have  long  disappeared  from  the 
market,  and  the  type  survives 
^  mainly  in  the  Bischoff,  which  is 
*'  specially  designed  for  small  powers, 
•>  mostly  under  half-horse.  It  is  an 
E  •  exceedingly  ingenious  little  engine, 
So  and  presents  many  interesting  pe- 
3  culiarities. 

Fig.  29  is  a  side  elevation,  part 
8  in  section ;  fig.  30  a  section  arranged 
'I  to  explain  the  valve  action.  In  both 
figures  the  similar  parts  are  marked 
with  similar  letters.  There  is  no  at- 
tempt to  gain  economy  by  attention 
to  theory  ;  the  aim  is  to  get  a  small 
workable  engine  with  the  least  possible  complication.  In  this  it  is 
very  successful.  To  avoid  the  complication  of  a  water-jacket, 
the  cylinder  and  piston  are  so  arranged  that  heating  is  allowable. 
The  engine  is  upright  and  very  peculiar  in  appearance,  the 
cvlinder  has  cast  on  it  a  number  of  radiating  ribs,  which  by 


Gas  Engines  of  Different  Types  in  Practice         133 

contact  with  the  air  cause  conduction  of  the  heat  more  rapidly 
than  would  otherwise  occur.  The  temperature,  however,  becomes 
very  high,  and  provision  is  made  to  prevent  injury  to  the  piston. 
It  is  fitted  loosely  to  the  cylinder  and  has  no  rings,  the  connecting 


FIG.  29. 

Bischoff  Engine. 


FIG.  30. 


rod  arrangement  is  seen  in  the  figure  (29) ;  it  takes  the  thrust  of  the 
explosion  in  tension,  and  almost  without  side  pressure  upon  the 
guide.  Any  side  pressure  upon  the  guide  is  quite  prevented  from 
reaching  the  piston,  and  it  consequently  is  never  rubbed  against 


134  The  Gas  Engine 

the  cylinder.  The  pressure  of  the  explosion  is  so  slight  that  the 
leakage  is  not  serious  even  without  rings.  The  piston  moves  up, 
taking  in  the  charge,  the  air  through  the  valve  i,  fig.  30,  which  is 
simply  a  piece  of  sheet  rubber  backed  by  a  thin  iron  disc.  The 
pressure  of  the  air  opens,  and  the  explosion  closes  it ;  the  valve  2, 
fig.  29,  similarly  made  but  smaller,  admits  the  gas ;  the  mixture 
does  not  form  till  the  gases  have  passed  the  point  3,  fig.  30 ; 
therefore  the  explosion  does  not  spread  back  to  the  valves. 
When  the  piston  gets  to  the  point  4,  it  crosses  a  small  aperture  5 
covered  by  a  light  hanging  valve  ;  a  flame  burning  outside  in  the 
flame  chamber  is  drawn  in.  The  explosion  then  occurs,  and  the 
pressure  at  once  closes  all  valves  and  propels  the  piston.  On  the 
return  stroke,  the  piston  valve  7  opens  to  the  exhaust  pipe  8,  at  the 
same  time  closing  the  passage  to  the  air  admission  valves.  The 
cylinder  proper  requires  no  lubrication ;  the  guide  requires  a 


Scale  i  in.  =  24  Ibs.  :  3*"  diam.  of  cylinder  \  \\\  ins.  stroke. 

FIG.  31. 
Diagram  from  i-man  power  Bischoff  Engine  ;   112  revs,  per  min.     \Cterk.) 

little,  but  the  projection  of  the  cover  and  a  draining  hole  prevent 
accumulation  and  overflow  of  oil  into  the  cylinder.  This  pre- 
caution is  very  necessary,  because  of  the  high  temperature  of  both 
piston  and  cylinder;  without  it  speedy  charring  and  choking 
up  of  the  cylinder  would  result.  The  arrangements  are  crude 
and  the  engine  is  somewhat  noisy,  but  it  is  very  reliable,  and  suits 
the  purpose  for  which  it  is  designed  exceedingly  well. 

Diagrams  and  Gas  Consumption. — The  diagram  is  very  similar 
to  the  Lenoir.  Fig.  31  is  a  diagram  taken  from  a  i-man  power 
engine  by  the  author. 

The  consumption  is,  as  might  be  expected,  rather  higher  than 
Lenoir.  According  to  tests  made  at  the  Stockport  Exhibition  it 
uses  120  cubic  feet  per  actual  horse  power  per  hour. 


Gas  Engines  of  Different  Types  in  Practice         135 

TYPE  (!A). 

Free  Piston  Engines. — The  very  high  consumption  of  gas 
common  to  the  engines  described  prevented  their  extended  use, 
and  set  inventors  to  work  to  produce  some  method  which  would 
give  better  results.  It  was  very  obvious  that  there  was  a  large 
loss  of  heat ;  the  trouble  with  cylinders  and  pistons  made  this 
abundantly  evident.  Devices  proposed  for  increasing  power  by 
the  injection  of  water  spray,  and  steam,  in  various  ways  failed 
to  produce  good  effect  except  in  aiding  lubrication.  The  inventors 
of  the  day  seem  to  have  reasoned  somewhat  in  this  fashion.  The 
force  generated  by  an  explosion  of  gas  and  air  is  an  exceedingly 
evanescent  one,  a  high  pressure  is  produced,  but  it  lasts  only  for 
a  very  short  time ;  if  work  is  to  be  obtained  before  loss  by 
cooling  absorbs  all  the  heat,  it  must  be  done  rapidly.  The 
reason  why  the  Lenoir  and  Hugon  engines  give  so  poor  a  result 
is  a  too  slow  movement  of  piston  after  the  explosion.  Therefore, 
if  a  method  can  be  devised  permitting  greater  piston  velocity, 
better  economy  will  be  obtained.  In  this  reasoning  there  was 
considerable  truth.  It  has  been  already  proved  that  the  shorter 
the  time  of  contact  between  the  charge  after  explosion  and  the 
enclosing  walls,  the  greater  will  be  the  efficiency  of  the  gas  in  the 
mixture.  But  this  only  holds  within  certain  limits.  If  the  expan- 
sion is  too  rapid  before  explosion  is  complete,  then  a  loss  instead 
of  a  gain  will  occur ;  the  expansion  should  not  commence  till 
maximum  pressure  is  attained  or  it  will  cause  a  loss  of  pressure. 
Indeed,  it  is  quite  conceivable  that  in  engines  of  the  Lenoir  type, 
the  expansion  might  be  so  rapid,  relatively  to  the  rate  of  explosion, 
that  no  increase  of  pressure  at  all  resulted  ;  in  which  case  no  power 
whatever  would  be  obtained.  The  gain  then  to  be  expected 
arises  from  rapid  expansion  after  complete  explosion.  This  has 
been  carried  out  by  several  inventors  by  the  free  piston  method. 
Instead  of  expending  the  force  of  the  explosion  upon  a  piston 
rigidly  connected  to  a  crank,  the  piston  is  allowed  free  movement. 
The  explosion  launches  it  against  the  atmosphere  ;  it  acquires 
considerable  velocity,  which  is  expended  in  compressing  the 
exterior  atmosphere,  that  is,  in  producing  a  vacuum  in  the  cylinder. 
When  all  the  energy  of  motion  is  expended,  the  piston  comes  to 


136  The  Gas  Engine 

rest,  and  the  atmospheric  pressure  forces  it  back  again.  So  soon 
as  the  return  movement  commences,  a  clutch  contrivance  engages 
the  shaft  and  drives  it.  Engines  of  type  i  A  may  be  described  as — 

Engines  using  a  gaseous  explosive  mixture  at  atmospheric 
pressure  before  explosion  ;  the  explosion  acting  on  a  piston  free  to 
move  without  connection  with  the  crank  shaft,  the  velocity  being 
absorbed  by  the  formation  of  a  vacuum.  The  power  is  given  to 
the  shaft  on  the  return  stroke  under  the  pressure  of  the  atmo- 
sphere. 

As  has  been  stated  in  the  historical  sketch,  the  first  to  propose 
this  kind  of  engine  were  Barsanti  and  Matteucci,  1857,  but  the 
difficulties  were  not  sufficiently  overcome  until  the  invention  of 
Otto  and  Langen,  1866. 

Otto  and  Langen  Engine. — This  engine  consists  of  a  tall  vertical 
cylinder  surrounded  by  a  water  jacket ;  in  it  works  a  piston  which 
carries  a  rack  instead  of  a  piston  rod;  the  mouth  of  the  cylinder  is 
open  to  the  atmosphere.  Across  the  top  of  the  cylinder  is  carried 
the  fly-wheel  shaft  ;  it  cannot  be  called  the  crank  shaft  because 
there  is  no  crank.  On  the  shaft  there  is  a  toothed  wheel  which 
engages  the  teeth  of  the  rack  ;  it  runs  freely  on  the  shaft  while  the 
piston  is  on  its  upward  stroke,  but  by  an  ingenious  clutch  arrange- 
ment it  grips  the  shaft  when  the  piston  moves  down.  The  shaft  is 
therefore  free  to  rotate  in  one  direction  and  the  piston  is  free  to 
move  up  without  restraint,  but  in  moving  down  it  gives  the  impulse. 
The  shaft  is  carried  on  bearings  bolted  to  the  top  of  the  cylinder, 
which  forms  a  strong  and  convenient  column  for  carrying  the 
mechanism  required  to  accomplish  the  cycle  of  the  engine.  At 
the  lower  end  of  the  column  is  placed  a  slide  valve  which  performs 
the  treble  duty  of  admitting,  igniting,  and  discharging.  It  is  driven 
from  an  intermediate  shaft,  intermittently,  as  determined  by  the 
governor  of  the  engine.  When  working  at  full  load,  the  move- 
ment of  a  small  crank  actuated  from  the  shaft,  lifts  the  rack  and 
piston  through  some  inches,  taking  in  the  charge  through  the  slide 
valve,  which  then  moves  further  and  brings  in  the  igniting  flame. 
The  explosion  ensues  and  shoots  up  the  piston  with  consider- 
able velocity,  the  pressure  rapidly  falls  by  expansion  and  soon 
gets  to  atmosphere.  The  piston  however  has  been  moving  freely 
and  therefore  has  done  no  work  ;  all  the  energy  of  the  explosion, 


Gas  Engines  of  Different  Types  in  Practice         137 

however,  has  been  given  to  it.  The  piston  has  the  energy  of  explosion 
in  the  form  of  velocity  ;  it  moves  on,  the  pressure  beneath  it  falling 
below  atmosphere  until  all  its  energy  of  motion  is  absorbed  in 
forming  the  vacuum.  When  this  occurs  it  ceases  its  upward  flight 
and  returns,  the  outer  atmosphere  driving  it  back,  and  as  the  clutch 
has  engaged  the  shaft,  an  impulse  is  given.  The  actual  work  is 


FlG.  32. — Otio  and  Langen  Engine  (vertical  section). 

therefore  done  by  the  atmosphere  on  the  down  stroke,  the  explosion 
being  spent  in  obtaining  energy  in  a  form  conveniently  applic- 
able. If  no  cooling  of  the  hot  gases  occurred  upon  the  down 
stroke  the  compression  line  would  return  to  the  point  where  the 
expansion  line  touched  atmosphere  ;  then  the  exhaust  valve  would 
open  and  the  gases  would  be  discharged  at  atmospheric  pressure- 


138  The  Gas  Engine 

In  that  case  the  work  done  by  the  atmosphere  and  weight  of  piston 
on  the  downward  stroke  would  exactly  equal  the  energy  of  the  ex- 
plosion while  falling  by  expansion  to  atmospheric  pressure.  But  the 
cylinder  does  cool  the  gases  while  on  the  upward  and  downward 
stroke,  so  that  the  expansion  line  does  not  return  upon  itself  ;  the 
amount  of  fall  below  the  expansion  line  is  gain  and  is  added  to  the 
energy  of  the  explosion  just  as  the  condenser  adds  to  the  efficiency 
of  the  expansion  of  steam.  The  exhaust  gases  are  expelled  by  the 
piston  and  a  new  stroke  is  commenced.  At  full  power  the  piston 
makes  about  30  strokes  per  minute,  the  shaft  rotating  about  90 
revolutions  per  minute.  The  governor  of  the  engine  is  so  arranged 
that  when  the  speed  becomes  too  great,  a  lever  disengages  a  pawl 
from  a  ratchet  and  disconnects  the  small  crank  lifting  the  piston. 
The  charge  is  not  taken  in  till  the  speed  falls,  and  then  the  pawl 
is  again  allowed  to  connect  the  small  crank  to  the  main  shaft. 
The  ignition  slide  gets  its  motion  from  the  small  crank  shaft,  so  that 
it  is  arrested  or  moved  along  with  the  piston.  The  piston 
remains  at  the  bottom  of  the  stroke  till  it  is  wanted  for  another 
explosion. 

Fig.  9,  p.  21,  shows  the  general  arrangement  of  the  engine,  and 
fig  32  is  a  vertical  section  showing  the  clutch  and  section  of  the  slide 
valve.  Fig.  33  is  an  elevation,  part  in  section. 

A  is  the  cylinder  ;  B  is  tht;  piston  to  which  is  attached  the 
rack  c  ;  D  the  toothed  wheel  containing  the  clutch  engaging 
the  rack  to  the  power  shaft.  The  rack  is  strongly  guided.  E  is 
the  fly-wheel  shaft  on  which  is  keyed  the  fly  wheel  F  and  the 
driving  pulley  G  ;  H  water  jacket  ;  i  the  port  for  inlet  of  the 
explosive  mixture  and  discharge  of  the  products  of  combustion  ; 
K  the  slide  valve  serving  to  admit,  to  ignite  the  charge  and  to  dis- 
charge the  products  of  combustion  ;  it  is  actuated  from  the  small 
shaft  i.  by  the  pin  M  ;  the  ratchet  N  and  the  pawl  o  connect  the 
small  shaft  to  the  main  shaft  when  requisite,  as  determined  by  the 
governor  lever  p. 

This  engine  is  the  result  of  great  care  and  labour  on  the  part 
of  the  inventors;  it  is  greatly  superior  in  economy  and  efficiency 
to  any  preceding  it,  and  its  only  fault  is  its  excessive  bulk  and 
weight  and  the  great  noise  made  by  it  when  in  action.  The  whole 
of  the  energy  of  the  explosion  being  expended  in  giving  the  piston 


Gas  Engines  of  Different  Types  in  Practice         139 

velocity,  just  as  in  a  cannon,  the  recoil  is  considerable.  So 
serious  is  it  that  none  but  the  very  smallest  engines  can  be  placed, 
upon  upper  floors  without  special  strengthening.  The  author  has 
seen  an  engine  at  work  where  the  vibration  produced  was  so  great 
that  props  were  put  under  the  engine  from  floor  to  floor  through; 
four  floors  to  get  a  solid  resistance  in  the  basement. 


FIG.  33.  — Otto  and  Langen  Engine  (elevation). 

In  other  cases  strong  iron  beams  placed  diagonally  at  the  angle 
of  a  stone  wall  carried  the  engine;  notwithstanding  these  precau- 
tions much  vibration  was  caused.  These  difficulties  did  not 
seriously  affect  the  sale  of  the  engine  for  small  powers,  but  they 
quite  prevented  it  being  made  for  powers  above  3-horse.  The 
clutch  also  is  a  matter  of  great  difficulty,  the  whole  power  of  the 


140 


The  Gas  Engine 


engine  passes  through  it  and  it  must  act  freely  and  instantaneously. 
The  faintest  back  lash  would  allow  the  accumulation  of  so  much 
velocity  by  the  return  that  even  a  strong  arrangement  would  be 
destroyed.  For  this  reason  the  pawl  and  ratchet  of  Barsanti  and 
Matteucci  failed  completely. 


Messrs.  Otto  and  Langen's  clutch  is  one  of  the  main  points  of 
their  invention  and  is  excellent.  It  is  shown  in  detail  at  fig.  34. 
The  part  a  is  keyed  to  the  shaft;  on  it  runs  the  part  b  carrying 


Gas  Engines  of  Different  Types  in  Practice         141 

the  teeth  engaging  the  rack.  So  long  as  b  moves  in  the  direction 
of  the  arrow  i,  or  is  stationary,  a  revolves  freely  with  the  shaft  in 
the  direction  2.  The  steel  slips  c,  c,  c,  c  are  wedge-shaped  on  the 
back,  so  is  the  interior  of  the  part  b  at  the  positions  d,  d,  d,  d. 
So  long  as  the  rack  is  stationary  or  ascending,  the  steel  rollers 
e,e,e,e,  run  freely  clear  of  the  inclined  surfaces  ;  immediately  the 
rack  moves  down  at  a  rate  greater  than  the  movement  at  A,  then 
the  rollers  are  firmly  wedged  between  the  two  inclined  surfaces 
and  the  steel  slips  c,  c,  c,  c  grip  the  part  a  firmly  and  drive  the 
shaft.  When  the  bottom  of  the  stroke  is  reached  the  wedges  loose 
again  and  the  piston  is  free. 

Diagrams  and  Gas  Consumption. — The  author  has  made  a  set 
of  experiments  upon  an  engine  of  2-horse  power  working  with 
Oldham  coal  gas. 

The  cylinder  is  12-5  inches  diameter  and  the  longest  stroke 
observed  was  40*5  inches.  Working  at  the  rate  of  28  ignitions 
per  minute,  the  indicated  power  was  2-9  horse,  and  the  gas  was 
consumed  at  the  rate  of  24-6  cubic  feet  per  i.h.p.  per  hour.  The 
brake  power  is  2  horse,  so  that  the  brake  consumption  is  at  the 
rate  of  36  cubic  feet  per  horse  power  per  hour.  This  does  not 
include  the  consumption  of  the  side  lights  which  is  in  all  12  cubic 
feet  per  hour. 

Fig.  35  is  a  diagram  from  the  engine  when  at  full  power. 

The  full  line  is  that  traced  by  the  indicator,  and  the  dotted 
line  is  the  real  line  of  pressures  marred  by  the  oscillation  of  the 
indicator  pencil. 

Professor  Tresca.  tested  a  half-horse  engine  at  the  Paris  Ex- 
hibition of  1867;  it  gave  0-456  brake  horse,  and  consumed  gas  at 
the  rate  of  44  cubic  feet  per  brake  horse  power  per  hour.  This  es- 
timate did  not  include  the  side  lights.  The  author's  test  gives  a 
better  result  than  that  of  M.  Tresca,  but  this  is  due  to  the  fact  of 
the  larger  engine  being  used.  It  is  probable  that  a  3-horse  engine 
would  give  a  consumption  of  about  30  cubic  feet  per  brake  horse 
power  per  hour. 

The  interest  excited  by  the  engine  at  the  time  of  its  first  trial 
was  naturally  great,  and  many  explanations  were  advanced  of  the 
cause  of  its  superiority  over  the  Lenoir  and  Hugon  engines. 
Strangely  enough  the  theory  of  the  engine  has  been  at  best  but 


142 


The  Gas  Engine 


imperfectly  stated  by  previous  writers;  some  indeed  have  fallen 
into  grave  error  respecting  its  action.  It  is  therefore  essential 
that  it  should  be  somewhat  fully  considered  here. 


The  name  by  which  it  is  most  widely  known  is  in  itself  mis- 
leading. Atmospheric  gas  engine  at  once  suggests  the  Newcomen 
steam  engine  and  further  suggests  the  substitution  of  flame  for 


Gas  Engines  of  Different  Types  in  Practice         143 

steam,  vacuum  in  both  cases  being  supposed  to  be  produced  by 
condensation.  In  the  steam  engine  the  name  truly  describes  the 
action  :  the  piston  is  drawn  up,  the  cylinder  filled  with  steam  at 
atmospheric  pressure  and  the  steam  condensed  by  a  water  jet ; 
then  the  atmosphere  presses  the  piston  down  and  gives  the  power. 

In  the  gas  engine  cooling  has  little  to  do  with  the  production 
of  the  vacuum  ;  the  vacuum  would  be  produced  and  the  engine 
would  act  efficiently  without  any  cooling  action  of  the  cylinder 
whatever.  The  diagram  fig.  35  proves  this  very  clearly.  While 
the  piston  is  moving  from  the  point  a  to  b  by  the  energy  stored 
up  in  the  fly  wheel,  the  charge  enters  the  cylinder;  at  b  the  piston 
pauses,  and,  the  igniting;  flame  being  introduced,  the  charge  ex- 
plodes, the  pressure  rises  to  54  Ibs.  per  square  inch  above  atmo- 
sphere. The  appearance  of  the  explosion  curve  does  not  indicate 
truly  the  rate  of  increase,  because  the  piston  is  completely  at  rest 
till  the  pressure  puts  it  in  motion.  The  piston  moves  up  impelled 
by  the  pressure  of  the  explosion;  as  it  moves  the  gases  beneath  it 
expand  and  therefore  the  pressure  falls.  At  the  point  d  the  pres- 
sure is  again  level  with  that  of  the  outside  atmosphere;  here  the 
explosion  ceases  to  impel  the  piston  and,  the  pressure  in  the  cy- 
linder falling,  the  atmosphere  presents  a  continually  increasing 
resistance.  But  while  the  piston  is  passing  from  the  point  b  to  d, 
the  pressure  has  been  falling  from  54  Ibs.  above  atmosphere  to 
atmosphere;  the  average  pressure  upon  it  through  this  distance  is 
1 2 '6  Ibs.  per  square  inch;  as  the  distance  is  1*3 feet  and  the  piston 
area  is  122-7  inches,  2010  ft.  pounds  have  been  expended  upon  it. 
What  becomes  of  this  work  ?  In  an  ordinary  engine  it  would  be 
communicated  to  the  crank,  and  if  no  load  were  on,  the  crank 
would  give  it  to  the  fly  wheel.  Here  there  is  no  crank  and  the 
piston  is  perfectly  free,  the  piston  alone  contains  the  energy  ;  its 
weight  has  been  raised  through  i  '3  feet  and  the  balance  of  the 
energy  is  stored  in  it  as  velocity  of  upward  movement. 

It  must  therefore  continue  to  move  up  till  its  energy  of  motion 
is  expended  in  compressing  the  atmosphere,  in  raising  the  piston, 
and  in  friction.  If  friction  did  not  exist  and  the  piston  was  indefi- 
nitely light,  then  the  portion  of  the  diagram  bed  would  be  equal 
in  area  to  the  portion  def,  that  is,  the  work  expended  by  the  ex- 
plosion in  giving  the  piston  velocity  would  be  equal  to  the  work 


144  The  Gas  Engine 

expended  by  the  atmosphere  in  bringing  it  to  rest  again.  Once 
at  rest  the  vacuum  produced  allows  the  piston  to  be  driven  down 
again,  this  time  to  give  up  its  energy  to  the  motor  shaft. 

As  the  piston  in  this  engine  weighs  116  Ibs.  the  work  spent  in 
raising  it  through  i^ft.  is  116  x  1*3  =  150*8  ft.  pounds;  deduct 
this  from  the  total  work;  and  2010  -  151  =  1859  ft.  Ibs.  is  the 
energy  of  motion  of  the  piston. 

The  relation  between  energy,  mass  and  velocity  is 

E  =  M7'2. 

2 

E  =  energy  in  absolute  units.  One  foot  pound  —  32  absolute 
units. 

M  =  mass  in  pounds. 

v  =  velocity  in  feet  per  second. 

The  velocity  is  therefore  v=  A  /  £^ 

V      M 

and 

E  =  1859  x  32  =  59488  absolute  units. 


M=    116 


The  velocity  of  the  piston  at  the  moment  when  the  explosion 
pressure  has  been  expended  and  the  internal  and  external  pressures 
exactly  balance  is  32  ft.  per  second  or  1560  ft.  per  minute ;  at  no 
point  of  the  stroke  in  any  ordinary  engine,  steam  or  gas,  is  such  a 
high  piston  speed  possible.  This  explains  the  recoil  of  the  engine. 
But  this  is  not  the  average.  The  piston  has  attained  32  feet  per 
second  after  moving  through  i  -3  feet ;  the  time  taken  to  move  that 

distance  is  /  =   ,  / 2S  when  /  =  time  in  seconds. 

V     v 

3  =  space  passed  through. 
z<  =  velocity, 
and  s  =  i  -3  feet  v  =  3  2  feet. 

=  0-28  second. 
32 

The  piston  has  taken  0*28  second  to  move  through  the  i'3 
feet  ;  its  average  velocity  during  the  action  of  the  explosion  is 


Gas  Engines  of  Different  Types  in  Practice         145 

therefore  4-64  feet  per  second  or  278  feet  per  minute.  This, 
although  high,  is  not  greatly  in  excess  of  that  used  in  the  Lenoir 
and  Hugon  engines.  It  is  less,  indeed,  than  the  average  piston 
speed  now  used  in  modern  compression  engines,  300  to  400  feet 
per  minute  being  common.  If  no  cooling  by  the  cylinder  occurred, 
the  line  cdf  would  be  adiabatic,  and  the  return  line  fg  would 
coincide  with  the  expansion  line  df\  the  portion  of  the  vacuum 
diagram  def'is  due  solely  to  the  energy  of  the  explosion,  the  part 
dfg  is  due  to  the  cooling  of  the  gases.  If  cooling  did  not  act  at 
all,  the  area  bed  would  be  greater,  and  therefore  d ef,  which  is  its 
equivalent,  would  also  be  greater,  that  is,  the  vacuum  produced 
would  be  greater  if  no  cooling  whatever  existed. 

The  theory  of  its  action  generally  held  at  the  time  of  M.  Tresca's 
experiments  seems  to  have  been  as  follows  : 

The  work  of  the  explosion  consists  simply  in  pushing  up  the 
piston  and  filling  the  space  behind  it  with  flame,  which  flame  is 
cooled  by  contact  with  the  cylinder,  and  a  vacuum  results.  The 
flame  is  considered  as  analogous  to  steam,  and  the  cooling  as 
similar  to  condensation  as  in  the  Newcomen  engine.  The  inven- 
tors of  the  engine  seem  to  share  this  erroneous  idea ;  certainly 
M.  Tresca  did,  as  in  his  report  upon  the  engine  he  says  :  '  There 
is,  therefore,  between  the  older  machines  and  the  new  one  this 
difference  of  principle,  that  the  pressure  in  the  cylinder  can  never 
descend  below  the  atmospheric  during  the  upward  stroke.  The 
negative  force  of  the  atmospheric  pressure,  useless  as  it  was,  be- 
comes utilisable.  .  .  .'  He  clearly  considered  that  the  pressure 
during  the  upward  stroke  was  expended  only  in  lifting  the  pistcn 
through  a  certain  height,  and  as  soon  as  it  fell  to  atmosphere  the 
piston  stopped  and  the  cooling  caused  a  vacuum,  the  work  being  done 
by  the  falling  of  the  piston  and  the  pressure  of  the  exterior  air. 
If  the  cooling  really  caused  the  vacuum,  the  diagram  would  be 
quite  different ;  instead  of  the  pressure  touching  atmosphere  at  the 
point  d,  it  would  not  touch  till  the  point  e  at  the  end  of  the  stroke. 
The  pressure  would  then  abruptly  fall  to  /  and  the  piston  would 
return.  M.  Tresca  observed  that  the  pressure  did  fall  below 
atmosphere  before  the  end  of  the  stroke,  but  he  considered  it  as  a 
defect.  '  In  reality  the  piston  rises  in  virtue  of  the  swiftness  ac- 
quired to  beyond  the  position  at  which  there  would  be  equilibrium 


146  The  Gas  Engine 

between  the  interior  and  exterior  pressure.  But  that  one  small 
loss  of  power  is  amply  compensated  for  by  the  atmospheric  power 
of  the  downward  stroke.' 

The  very  principle  of  the  engine  really  depends  upon  this  fall 
of  pressure  which  was  considered  by  M.  Tresca  a  defect ;  it  is  the 
only  way  to  store  up  the  power  of  the  explosion  so  that  it  may  be 
available  on  the  downward  stroke.  If  the  pressure  did  not  fall, 
the  piston  would  require  to  be  projected  10-5  feet  into  the  air  to 
absorb  the  energy  of  the  explosion  in  its  mass  alone  ;  by  the  fall 
of  pressure  it  is  absorbed  with  the  smaller  movement  of  i'3  feet 
The  only  part  of  the  diagram  due  to  cooling  is  the  part  dfg,  not 
more  than  one- fifth  of  the  total  area  representing  work  done  by 
the  engine. 

The  superior  economy,  it  is  evident,  cannot  be  altogether  due  to 
greater  piston  velocity  ;  the  piston  velocity,  although  considerable, 
is  not  superior  enough  to  that  of  Lenoir  and  Hugon  to  account 
for  all  the  difference.  There  must  exist  other  points  of  dissimi- 
larity. In  the  Lenoir  type  of  engine  the  strokes  were  numerous 
and  the  gas  consumed  per  stroke  on  the  whole  smaller  than  in 
Otto  and  Langen  engines  of  equal  power  :  the  latter  used  few 
strokes  but  large  cylinders  ;  proportionally  the  cooling  surface 
exposed  was  thus  diminished.  Then  the  piston  is  at  rest  until 
the  explosion  puts  it  in  motion  ;  the  pressure  gets  time  to  rise  to 
its  maximum  before  the  piston  moves  and  expands  the  space. 
Maximum  pressure  is  attained  at  constant  volume  as  required  by 
theory  ;  at  the  same  time  the  piston  and  cylinder  remain  cool  be- 
cause of  the  infrequency  of  the  strokes.  The  entering  charge  is 
therefore  but  slightly  heated  before  explosion,  and  the  explosion 
gives  a  better  pressure  for  a  smaller  elevation  of  temperature. 

The  most  potent  cause  of  improvement,  however,  is  great  ex- 
pansion :  the  large  cylinders  allow  an  expansion  of  10  times  the 
volume  existing  before  explosion,  and  so  gain,  first  by  expanding 
to  atmosphere,  and  second  by  the  cooling  which  follows  the  further 
expansion.  A  comparison  of  the  actual  diagram  with  the  theo- 
retical reveals  some  interesting  peculiarities  which  seem  hitherto 
to  have  escaped  observation.  The  maximum  pressure  on  the 
diagram,  which  is  above  the  true  pressure,  fig.  35,  is  54  Ibs.  above 
atmosphere,  corresponding  to  a  temperature  of  1355°  absolute. 


Gas  Engines  of  Different  Types  in  Practice         147 

The  mixture  exploded  contains  i  volume  gas  and  7  volumes  air 
(Oldham  gas)  ;  if  all  the  heat  present  had  been  evolved  by  the 
explosion  the  pressure  should  have  been  168  Ibs.  above  atmo- 
sphere. At  the  maximum  pressure  only  32  per  cent,  of  the  heat  has 
been  evolved,  leaving  68  per  cent,  to  be  evolved  during  the  ex- 
pansion. The  line  c  d  is  very  much  above  the  adiabatic,  so  much 
so  that  the  curve  c  d  is  nearly  isothermal,  the  temperature  at  d  only 
becoming  1305°  absolute  instead  of  733°,  which  it  should  be  if  adia- 
batic. The  heated  gases  are  therefore  gaining  heat  from  c  to  d,  and 
as  the  only  source  is  combustion,  it  follows  that  the  combination 
is  not  nearly  complete  at  the  maximum  pressure.  The  68  per  cent. 


109  a" 


.-1160° 


Scale  i  in.  =  24  Ibs.     Diluted  mixture,  gas  i  vol.,  air  12  vols. 


«  :? 

o 
k     2 


FIG.  36. — Diagram  from  2  h.  p.  Otto  and  Langen  Engine  (Clerk). 

of  the  total  heat  which  has  not  appeared  at  the  maximum  pressure 
is  appearing  during  the  expansion.  The  combustion  seems  to  be 
nearly  complete  at  the  point  d  as  the  line  de  behaves  as  if  cooling  ; 
if  adiabatic,  the  temperature  at/ should  be  961° — it  is  870°.  During 
compression  to  atmosphere  again,  the  temperature  remains  con- 
stant at  870°  ;  the  cooling  power  of  the  cylinder  is  equal  only  to 
preventing  increase  which  would  otherwise  occur. 

This  effect  is  more  evident  with  a  more  dilute  mixture.    Fig.  36 
is  a  diagram  taken  by  the  author  from  the  same  engine,  but  using 

L  2 


148 


The  Gas  Engine 


a  mixture  containing  i  volume  of  gas  and  1 2  volumes  of  air.  Here 
the  maximum  pressure  is  only  17  Ibs.  per  square  inch  above  atmo- 
sphere. With  complete  evolution  of  heat  it  should  be  103  Ibs. ;  the 
maximum  pressure  in  this  case  only  accounts  for  24  per  cent,  of  the 
heat  known  to  be  present,  leaving  76  per  cent,  to  be  evolved  during 
expansion.  The  diagram  affords  the  most  ample  proof  that  the 


Absolute 


10  20  30  40  50  60  70 

Percentage  of  stroke. 


9o 


So 


70 


Co 


90  100 


FIG.  37.— Otto  and  Langen  Engine.     Free  Piston. 

combustion  is  proceeding,  the  falling  line  shows  a  steady  increase 
of  temperature  to  the  very  end  of  the  stroke.  The  temperatures 
are  marked  upon  the  diagram  at  the  successive  points  ;  taking  the 
temperature  at  the  point  b  as  290°  absolute,  the  points  c,  d,  e,f,g,  h 
are  respectively  780°,  936°,  1092°,  1107°,  1160°,  and  1225°,  show- 
ing a  steady  increase  throughout  the  whole  expansion  line,  right 


Gas  Engines  oj  Different  Types  in  Practice         149 

to  the  end  of  the  stroke.  The  consumption  of  gas  per  indicated 
HP  rises  very  much  in  consequence,  amounting  to  about  37  cubic 
feet  per  IHP  hour.  The  power  at  the  same  time  falls,  so  that 
the  30  explosions  per  minute  are  required  to  keep  the  engine  going 
without  load  at  53  revolutions  per  minute.  The  cooling  during 
compression  is  so  slow  that  the  temperature  falls  only  from  1225° 
to  96  7°,  from  the  point  h  to  k.  All  the  published  diagrams  examined 
by  the  author  show  this  peculiar  effect.  Fig.  37  is  a  diagram  pub- 
lished by  Mr.  F.  W.  Crossley.  Taking  80  Ibs.  as  the  maximum  pres- 
sure, which  seems  somewhat  higher  than  is  warranted  by  the  dia- 
gram, the  oscillation  of  the  indicator  has  been  so  excessive,  the  cor- 
responding temperature  is  1873°  absolute  :  the  expansion  line  c  d  if 
adiabatic  would  give  at  the  point  d  a  temperature  of  1090°,  the 
actual  temperature  is  1044°.  Within  the  limits  of  error  they  may 
be  considered  the  same  ';  there  is  therefore  combustion  going  on 
from  c  to  d  also.  At  the  point  e  the  temperature  is  788°  ;  if  adia- 
batic it  should  be  667°.  It  is  quite  evident  that  the  whole  of  this 
expansion  curve  is  above  the  adiabatic  ;  in  the  earlier  part  of  the 
diagram  the  oscillation  causes  uncertainty,  but  in  the  latter  part 
the  measurement  is  true  enough. 

The  compression  line  eg\s  almost  isothermal,  788°  at  e,  cooling 
to  737°  at# 

Fig.  38  is  a  diagram  by  Releaux  taken  from  Schottler. 

It  is  manifestly  wrong,  as  the  vacuum  part  is  much  too  small 
and  the  maximum  temperature  is  higher  than  has  ever  been  ob- 
tained by  any  explosion  of  gas  and  air,  but  if  taken  as  relatively 
correct  the  expansion  line  is  much  above  the  adiabatic. 

From  his  study  upon  the  explosion  of  gas  and  air  mixtures  in 
closed  vessels,  the  reader  will  be  prepared  to  find  that  only  a 
portion  of  the  total  heat  present  is  evolved  by  explosion  in  any 
gas  engine.  That  is,  the  explosion  maximum  pressure  never  accounts 
for  the  whole  heat  present  as  inflammable  gas  ;  a  portion  is  in  some 
manner  suppressed  and  is  not  evolved  till  long  after  the  moment 
of  complete  explosion.  Combustion  is  not  completed  till  con- 
siderably after  the  completion  of  explosion. 

He  will  be  unprepared,  however,  for  such  diagrams  as  figs.  35 
and  36,  where  the  maximum  pressure  represents  only  0-32  and  0*24 
of  the  heat  present,  and  o-68  and  076  are  evolved  during  the  forward 


150 


The  Gas  Engine 


stroke  while  the  pressure  is  falling.  The  explanation  is  simple. 
The  case  is  quite  different  from  that  of  the  closed  vessel  or  where 
the  piston  is  connected  to  a  crank.  As  soon  as  the  pressure  of 
the  explosion  becomes  great  enough,  the  piston  at  once  moves  out 
and  prevents  further  increase  of  pressure.  The  slower  the  rate  at 
which  the  mixture  inflames,  the  greater  will  be  the  apparent  sup- 
pression of  heat ;  thus  the  mixture  i  volume  gas  7  air  takes,  in  the 
closed  vessel,  0-06  second  to  complete  the  explosion,  but  before  this 


Stroke. 

FIG.  38.— Otto  and  Langen. 

time  has  elapsed  the  piston  is  in  rapid  motion  reducing  the  pressure 
before  complete  explosion.  To  get  the  maximum  pressure  it 
would  be  necessary  to  prevent  it  from  moving  till  the  explosion 
was  complete.  The  weaker  mixture  takes  0^25  second  to  complete 
the  explosion,  and  so  in  diagram,  fig  36,  the  temperature  actually 
rises  throughout  the  whole  stroke. 

A  heavier  piston  would  be  longer  in  starting  under  the  pressure 
of  the  explosion  and  would  so  allow  a  high  pressure  to  be  attained 


Gas  Engines  of  Different  Types  in  Practice         157 


FREE  PISTON1 


with  a  given  mixture.  This  explains  Mr.  Crossley's  remark  in  reading 
his  paper,  that  heavy  piston?  gave  a  more  economical  result  than 
light  ones. 

Gilles  Engine. — The  great 
success  of  the  Otto  and  Lan- 
gen  engine  occasioned  many 
attempts  to  improve  upon 
it.  Its  merits  and  its  faults 
were  equally  evident.  The 
recoil  of  the  engine  at  every 
stroke  was  exceedingly  trouble- 
some, and  the  noise  of  the  rack 
and  clutch  could  be  heard  at 
a  long  distance.  Gilles  of  Co- 
logne invented  an  engine  in- 
tended to  retain  the  economy 
while  reducing  the  noise.  In 
it  there  are  two  pistons,  one 
free,  the  other  connected  to  the 
crank  in  the  usual  way.  The 
free  piston  being  at  the  bottom 
of  its  stroke  and  close  to  the 
crank  piston,  the  latter  moves 
a  portion  of  its  stroke,  taking 
in  the  explosive  charge  ;  at  a 
suitable  position  ignition  oc- 
curs and  the  free  piston  is 
driven  in  one  direction  while 
the  other  completes  its  out- 
stroke.  A  vacuum  is  pro- 
duced between  the  pistons, 
and  the  free  piston  rod  being 
gripped  by  a  clutch  is  kept  in 
its  extreme  position  till  the 
main  piston  returns  under  the 

pressure   of    the   atmosphere.    FIG.  39. -Gilles  Engine.     Free  Piston. 

The  clutch  is  then  released  and 

the  free  piston  falls,  expelling  the  exhaust  gases.     Fig.  39  is  a 


i  $2  The  Gas  Engine 

section  of  the  engine.  It  is  unnecessary  to  give  further  descrip- 
tion, as  the  engine  was  not  so  economical  or  so  simple  as  the 
earlier  one. 

TYPE   II. 

In  engines  of  this  kind  compression  is  used  previous  to 
ignition,  but  the  ignition  is  so  arranged  that  the  pressure  in  the 
motor  cylinder  does  not  become  greater  than  that  in  the 
compressing  pump.  The  power  is  generated  by  increasing 
volume  at  a  constant  pressure.  Engines  of  Type  II.  are  there- 
fore : 

Engines  using  a  mixture  of  inflammable  gas  and  air  compressed 
before  ignition  and  ignited  in  such  a  manner  that  the  pressure 
does  not  increase,  the  power  being  generated  by  increasing 
volume. 

These  engines  are  truly  slow  combustion  engines  ;  in  them  there 
is  no  explosion. 

The  most  successful  engine  of  the  kind  is  an  American  invention; 
although  proposed  in  1860  by  the  late  Sir  William  Siemens,  it  was 
never  put  into  practicable  workable  shnpe  till  1873,  when  the 
American,  Brayton  of  Philadelphia,  produced  his  well-known 
machine. 

Messrs.  Simon  of  Nottingham  introduced  it  into  this  country  in 
1878.  They  added  one  thing  only  of  doubtful  utility — that  is,  the 
use  of  steam  raised  in  the  water  jacket  as  auxiliary  to  the  flame  in 
the  motor  cylinder. 

Brayton  Engine. — In  this  engine  there  are  two  cylinders,  com- 
pressing pump  and  motor.  The  charge  of  gas  and  air  is  drawn 
into  the  pump  on  the  out-stroke  and  compressed  on  the  return 
into  a  receiver  ;  the  pressure  usual  in  the  receiver  varies  from  60 
to  80  Ibs.  per  square  inch  above  atmosphere.  The  motor  cylinder 
takes  its  supply  from  the  receiver  but  the  mixture  is  ignited  as  it 
enters,  a  grating  arrangement  preventing  the  flame  from  passing 
back;  the  mixture,  in  fact,  does  not  enter  the  motor  cylinder  at  all ; 
what  enters  it,  is  a  continuous  flame.  At  a  certain  point  the  supply 
of  flame  is  cut  off  and  the  piston,  moving  on  to  the  end  of  its  stroke, 
expands  the  volume  of  hot  gases  to  nearly  atmospheric  pressure 
before  discharge. 


Gas  Engines  of  Different  Types  in  Practice         153 

Fig.  40  is  an  external  view  of  the  engine.  Figs.  41  and  42  are 
sections  of  the  motor  and  pump  cylinders.  The  action  is  as  fol- 
lows : — The  engine  is  single  acting,  receiving  one  impulse  for  every 
revolution  ;  like  all  gas  engines  it  depends  upon  the  energy  stored 
up  in  the  fly  wheel  to  carry  it  through  those  parts  of  its  cycle 


FIG.  40. — Brayton  Petroleum  Engine. 

where  the  work  is  negative.  The  two  cylinders  are  inverted  and 
are  attached  to  a  beam  rocking  beneath  them,  by  connecting  rods. 
The  beam  is  prolonged  and  connected  to  the  crank  above  it  by  a 
rod  ;  both  cylinders  are  single-acting  and  the  pistons  are  of  the 
trunk  kind.  Both  pump  and  motor  cylinders  are  of  the  same 


154  The  Gas  Engine 

diameter,  but  the  pump  is  only  half  the  stroke  of  the  motor.  The 
valves  are  actuated  from  a  shaft  running  at  the  same  rate  as  the 
main  shaft  and  driven  from  it  by  bevel  wheels.  There  are  four 
halves,  all  of  the  conical  seated  kind — two  upon  the  motor, 
admission  and  discharge,  two  upon  the  pump  cylinder,  admission 
and  discharge.  The  admission  and  discharge  valves  upon  the 
motor  are  actuated  from  the  auxiliary  shaft  by  levers  and  cams,  so 
is  the  pump  inlet.  The  pump  discharge  valve  is  automatic,  rising 
at  the  proper  time  by  the  pressure  of  compression.  During  the 
down-stroke  the  pump  takes  in  the  charge  of  gas  and  air,  forcing 
it  on  the  up-stroke  into  the  receiver.  From  the  receiver  it  is  led  to 
the  power  cylinder,  passing  by  the  inlet  valve  through  a  pair  of 
perforated  brass  plates  with  wire  gauze  placed  between  them. 
Through  this  diaphragm  a  small  stream  of  mixture  is  constantly 
passing  into  the  motor  cylinder;  before  the  engine  is  started,  a 
plug  is  withdrawn  and  the  current  lighted  ;  a  constant  flame  is 
therefore  burning  under  the  diaphragm.  The  mixture  enters  the 
cylinder  through  this  flame,  lighting  as  it  enters  ;  at  all  times 
during  the  exhaust  part  of  the  stroke,  as  well  as  the  admission,  the 
stream  of  entering  mixture,  from  the  receiver,  keeps  up  a  small 
constant  flame  which  is  augmented  at  the  beginning  of  the  stroke, 
so  as  to  fill  the  cylinder  entirely,  when  the  admission  valve  is 
opened.  When  the  admission  valve  is  closed,  the  bye-pass  keeps 
the  flame  fed  with  sufficient  mixture  to  keep  it  alight.  The 
pressure  in  the  cylinder  thus  never  exceeds  that  in  the  reservoir 
and  the  mixture  burns  quietly  without  spreading  back. 

Figs.  40,  41  and  42. — A  is  the  motor  cylinder  ;  B  the  pump  ; 
the  beam  and  connections  require  no  lettering ;  c  is  the  pump 
inlet  valve  (the  pump  discharge,  which  is  an  ordinary  lift  valve,  is 
not  seen  in  fig.  42,  but  is  lettered  D  in  fig.  40) ;  E  the  motor  inlet ; 
F  the  igniting  plug  which  is  withdrawn  when  the  flame  is  to  be  lit 
before  starting  the  engine  (see  fig.  82) ;  G  is  the  grating  in  section 
(see  fig.  41);  H  the  exhaust  valve;  the  levers  and  cams  are 
sufficiently  indicated  on  the  drawing  ;  the  small  pipe  and  stop- 
cock i  (fig.  40)  communicates  at  all  times  with  the  reservoir  and 
supplies  the  constant  flame  with  mixture.  The  engine  worked 
well  and  smoothly ;  the  action  of  the  flame  in  the  cylinder  could 
not  be  distinguished  from  that  of  steam,  it  was  as  much  within 


Gas  Engines  of  Different  Types  in  Practice         155 

control  and  produced  diagrams  quite  similar  to  steam.  The  flame 
grating  was  the  weak  point ;  it  stood  exceedingly  well  for  a  time,  but 
if  by  any  accident  the  gauze  was  pierced  in  cleaning,  the  flame 
went  back  into  the  reservoir  and  exploded  all  the  mixture — the 
engine,  of  course,  pulled  up  as  the  constant  flame  having  no  supply 


PIG.  41.— Brayton  Engine. 
Section  of  Motor  Cylinder. 


FlG.  42. — Brayton  Engine. 
Section  of  Pump  Cylinder. 


was  extinguished.  This  accident  became  so  troublesome  that 
Mr.  Brayton  discontinued  the  use  of  gas  and  converted  his  engine 
into  a  petroleum  engine.  The  light  petroleum  was  pumped  upon 
the  grating  into  a  groove,  filled  with  felt,  the  compressing  pump 


I56 


The  Gas  Engine 


then  charged  the  reservoir  with  air  alone.  The  air  in  passing 
through  the  grating  carried  with  it  the  petroleum,  partly  in  vapour, 
part  in  spray;  the  constant  flame  was  fed  by  a  small  stream  of  air. 
The  arrangements  were,  in  fact,  precisely  similar  to  the  gas  engine, 
except  in  the  addition  of  the  small  pump  and  the  slight  alteration 
in  the  valve  arrangements.  The  difficulty  of  explosion  into  the 
reservoir  was  thus  overcome,  but  a  new  difficulty  arose — the 
cylinder  accumulates  soot  with  great  rapidity  and  the  piston 
requires  far  too  frequent  removal  for 
cleaning.  The  petroleum  pump  is  an 
exceedingly  clever  little  contrivance  ; 
fig.  43  shows  its  details.  The  amount 
of  petroleum  to  be  injected  at  each 
stroke  is  so  small  that  an  ordinary  force 
pump  with  clack  valves  would  be  un- 
certain. Brayton  gets  over  this  difficulty 
by  substituting  a  slide  valve  driven  from 
the  eccentric. 

The  plunger  of  the  pump  is  no  larger 
than  a  black-lead  pencil,  yet  it  dis- 
charges any  quantity,  from  a  single  drop 
per  stroke  up  to  full  throw,  with  un- 
erring certainty.  The  plunger  also  is 
driven  from  an  eccentric.  Both  eccen- 
trics are  in  one  piece  and  rotate  on  the 
end  of  the  auxiliary  shaft,  driven  by  a 
pawl  when  the  engine  is  in  motion  ;  to 
allow  of  starting,  the  pump  can  be 
moved  by  a  hand-crank  independently. 
To  start,  the  air  reservoir  is  filled,  if  not 
already  full,  by  turning  the  engine  round 
by  hand  ;  the  plug  F  is  then  withdrawn 
and  a  little  petroleum  thrown  upon  the  diaphragm  by  a  few  turns 
of  the  pump.  The  cock  i  on  the  small  pipe  is  then  opened  and 
a  stream  of  air  flowing  from  the  reservoir  vaporises  the  petroleum  ; 
it  is  lit  at  o,  and  the  flame  having  enough  air  for  combustion  retreats 
to  the  grating  and  remains  burning  within  the  cylinder.  The  plug 
is  then  inserted,  the  starting  cock  opened,  and  the  engine  starts. 


Fin.  43. 
Brayton  retroleum  Pump. 


Gas  Engines  of  Different  Types  in  Practice         157 

The  flame  remains  alight  during  the  whole  time  the  petroleum 
continues  to  be  supplied. 

The  valves  act  well  and  the  motor  cylinder  does  not  suffer 
from  the  action  of  the  flame  so  long  as  it  is  kept  reasonably 
clean.  If  the  soot,  however,  is  allowed  to  accumulate,  it  speedily 
cuts  up. 

Diagrams  and  Gas  or  Petroleum  Consumption. — Prof.  Thurston 
of  the  Stevens  Institute  of  Technology  tested  a  Brayton  gas  engine 
in  New  York  in  the  year  1873. 

The  following  extracts  are  from  his  report  : 

'  The  operation  of  the  engine  is  precisely  similar  in  the  action 
of  the  engine  proper  and  in  the  distribution  of  pressure  in  its 
cylinder,  to  that  of  the  steam  engine.  The  action  of  the  impelling 
fluid  is  not  explosive  as  it  is  in  every  other  form  of  gas  engine  of 
\vhich  I  have  knowledge. 

'  Upon  the  opening  of  the  induction  valve,  the  mixed  gases 
enter,  steadily  burning  as  they  flow  into  the  cylinder,  and  the 
pressure  from  the  commencement  of  the  stroke  to  the  point  of  cut 
off,  as  is  shown  by  the  indicator  diagrams,  is  as  uniform  as  that 
observed  in  any  steam  engine  cylinder.  The  maximum  pressure 
exerted  during  my  experimental  trial,  and  while  the  engine  was 
driving  somewhat  more  than  its  full  rated  power,  was  about  75  Ibs. 
per  square  inch  at  the  beginning  of  the  stroke,  gradually  dimin- 
ishing to  66  Ibs.  per  square  inch  at  the  point  of  cut-off,  where 
the  speed  of  the  piston  was  nearly  at  a  maximum,  and  then  de- 
clining in  accordance  with  the  law  governing  the  expansion  of 
gases. 

'  Complete  combustion  is  insured  by  thorough  mixture.  This 
is  accomplished  by  taking  the  illuminating  gas  and  air,  in  proper 
proportion,  into  the  compressing  pump  together,  and  the  mixture 
here  made  becomes  more  intimate  in  the  reservoir,  and  in  its  pro- 
gress towards  the  point  at  which  it  does  its  work.  The  constantly 
burning  jet  already  described  insures  prompt  ignition  on  entering 
the  cylinder. 

'.  .  .  the  engine  rated  at  5  HP  developed,  as  a  maximum, 
rather  more  than  its  rated  power.  Its  mean  power  during  the  test, 
as  determined  by  the  dynamometer,  was  3 '986  HP,  the  indicator 
showing  at  that  time  8-62  HP  developed  in  the  cylinder.  The 


I58 


The  Gas  Engine 


amount  of  gas  consumed  averaged  32-06  cubic  feet  per  indicated 
HP  per  hour. 

'The  excess  of  indicated  over  dynamometric  HP  is  to  be 
rtttributed  to  the  work  of  driving  the  compressing  pump  and  to  the 
friction  of  the  machine. 

'  The  greater  portion  of  this  appears  both  in  debit  and  credit 
side  of  the  account,  since,  although  expended  in  the  compressing 
pump,  it  is  restored  again  in  the  driving  cylinder.' 

The  consumption  of  32-06  cubic  feet  per  horse  hour  is  incor- 
rect ;  it  is  obviously  unfair  to  include  the  pump  diagram  in  the 
gross  power.  The  author  has  tested  an  engine  of  similar  con- 
struction and  dimensions  ;  he  finds  the  friction  of  the  mechanism 


Max.  press.  63  Ibs.  per  sq.  in. 
FlG.  44. 


FIG.  45.     Diagrams  from  BraytorTs  Gas  Engine. 

to  be  about  i -horse  ;  adding  this  number  to  the  dynamometric 
power  of  Prof.  Thurston,  the  legitimate  indicated  power  may  be 


taken  as  5  HP,  the  consumption  is  therefore 


5-62  x  32-06  _ 


5'° 


and  the  gas  per  brake  HP  per  hour  is  -1—  -  =  69-3.  These 

3-986 

numbers,  although  showing  improvement  upon  the  Lenoir  and 
Hugon,  prove  that  the  engine  was  much  inferior  in  economy  to 
the  Otto  and  Langen  engines. 

Mr.  H.  McMutrie,  Consulting  Engineer  at  Boston,  took  dia- 
grams from  an  engine  of  similar  dimensions  which  confirm  these 
results.  Fig.  44  is  the  diagram  taken  with  full  load,  fig.  45  the 
diagram  from  the  motor  with  no  load  on,  the  power  being  just 
sufficient  to  overcome  friction  and  pump  losses. 


Gas  Engines  of  Different  Types  in  Practice         150 


FULL  LOAD  DIAGRAM. 

Area  of  piston      .         .         ...         ,         .  50^26  sq.  ins. 

Speed  of  pi-ton  .         ,        ...         .        .  iSo  ft.  per  min. 

Mean  pressure 33  Ibs.  per  sq.  in. 

Pressure  in  reservoir 75'4  Ibs.  per  sq.  in. 

Initial  pressure  in  cylinder   .         ,        ...  68  Ibs.  per  sq.  in, 

Gross  pov\  er  developed        .         ,         .         .         .  9  H  P. 

No  LOAD  DIAGRAM. 

Speed  of  piston  .......  180  ft.  per  min. 

Mean  pres-ure     .......  18  Ibs.  per  sq.  in. 

Friction  and  other  resistance        ....  4 '87  HP. 

Net  available  power     .         .         .         .         .         .  9  —  4-87  =  4^13 

This  power  agrees  closely  with  the  actual  determination  by 
dynamometer. 

The  author  has  made  a  careful  trial  of  a  Brayton  petroleum 
engine  rated  at  5-horse.  The  engine  was  made  by  the  'New  York 
and  New  Jersey  Ready  Motor  Company  ;'  it  was  sent  to  Glasgow 
and  the  following  test  was  made  at  the  Crown  Ironworks  on  the 
2ist  and  22nd  February,  1878.  The  motor  cylinder  is  8  inches 
in  diameter  and  the  stroke  12  inches  ;  the  pump  cylinder  is  also 
8  inches  diameter  but  the  stroke  is  6  inches. 

Diagrams  were  taken  from  both  pump  and  motor  by  a  well- 
made  Richards'  indicator.  At  the  same  time  the  dynamometer 
was  applied  to  the  fly  wheel  fully  loading  the  engine,  readings  were 
taken  at  regular  intervals.  The  revolutions  were  recorded  by  a 
counter.  The  petroleum  used  was  measured  in  a  graduated  glass 
vessel . 

The  results  are  as  follows  : 

TK.ST  OF  BRAYTON  PETROLEUM  ENGINE.     (Clerk.} 

Petroleum  consumed  during  one  hour      .         .         .  i '378  gallons. 

Mean  speed  of  engine       .,.,.,  201  revs,  per  min, 

Mean  dynamometer  reading     .....  4^26  HP. 

Mean  pressure,  power  cylinder          .         .         .         .  31  Ibs.  per  sq.  in. 

Mean  pressure,  air  pump  .....  27^6  Ibs.  per  sq.  in. 

Piston  speed,  motor          ,         .         .         ,         .         .  201  ft.  per  min. 

Piston  spetd,  purrp          ......  loo's  ft.  per  IT  in. 

Power  indicated  in  motor          .         .         .         .         .  9*49  HP. 

Power  indicated  in  pump          .....  4'io  HP. 

Available  indicated  power        ,        .         .  5'39 


i6o 


The  Gas  Engine 


The  power  by  the  dynamometer  is  4'26-horse  ;  therefore  the 
mechanical  friction  of  the  engine  is  5-39  — 4-26=1-13  horse. 


Consumption  of  petroleum 
Consumption  of  petroleum 


•     °'255  galls,  per  IHP  per  hr. 

.  •  0*323  galls,  per  actual  HP  per  hr. 


Figs.  46  and  47  are  diagrams  from  the  motor  and  pump,  which 
are  fair  samples  of  those  taken.  It  will  be  observed  that  consider- 
able throttling  occurs  in  entering  the  motor  cylinder  ;  the  pump 
pressure  is  higher  than  the  reservoir  pressure,  and  the  motor  pres- 
sure is  lower,  so  that  a  double  loss  has  been  incurred.  The  prin- 
ciple of  the  engine  is  so  good  that  the  author  anticipated  better 
results.  Great  improvement  could  be  obtained  by  reproportioning 


60 


23 


Mean  pressure  30*2  Ibs.  per  sq.  in.     8  ins.  dia.  cylinder.     Stroke  12  ins.     200  revs,  per  min. 
FlG.  46. — Brayion  Petroleum  Engine.     Motor  Cylinder. 


n 


53 


65 


Mean  pressure  27*6  Ibs.  per  sq,  in.     8  ins.  dia.  cylinder.     Stroke  6  ins.     200  revs,  per  min. 
FlG.  47. — Brayton  Petroleum  Engine.     Pump  Cylinder. 

the  valves  and  air  passages  ;  they  are  in  this  engine  much  too  small 
and  cause  needless  resistance  and  loss.  The  maximum  pressure 
in  the  motor  cylinder  is  48  Ibs.  per  square  inch,  which  remains 
steadily  till  the  inlet  valve  shuts  at  four-tenths  ot  the  stroke  :  the 
pressure  then  slowly  falls  as  the  gases  expand,  the  exhaust  valve 
opening  at  about  ten  pounds  per  square  inch  above  atmosphere. 
The  average  available  pressure  upon  this  diagram  is  30-2  Ibs. 


Gas  Engines  of  Different  Types  in  Practice         161 

per  square  inch.  The  air  pump  shows  a  maximum  pressure  of 
65  Ibs.  per  square  inch,  the  reservoir  pressure  being  60  Ibs.  The 
average  resistance  is  27-6  Ibs.  per  square  inch  ;  as  the  pump  is  half 
the  stroke  of  the  motor  and  equal  to  it  in  area,  the  pressure  to  be 

deducted  is   -?—  =  i3'8  and  30-2  —  i3'8  =  16-4.     The  actual 
2 

available  pressure  actuating  the  engine  is  therefore  only  i6'4  Ibs. 
per  square  inch.  The  effect  of  the  clearance  in  the  pump  cylinder 
is  noticeable  upon  the  diagram  ;  the  air  inlet  valve  does  not  open 
till  one-tenth  of  the  down  stroke  is  completed. 

The  theoretic  efficiency  of  this  type,  with  a  maximum  tempera- 
ture of  1600°  C,  compression  of  60  Ibs.  per  square  inch  above 
atmosphere,  and  motor  cylinder  of  twice  the  pump  volume,  is  0-30  ; 
the  efficiency  of  the  gas  in  the  mixture  commonly  used,  i  volume  gas 
7  volumes  of  air,  is  0*40  (p.  113) ;  so  that  if  the  conditions  of  loss 
by  cooling  are  no  worse  than  in  the  author's  explosion  experiments, 
and  the  diagram  appeared  perfect,  the  actual  indicated  efficiency 
would  be  0-30  x  0-40  =  o-i2.  That  is,  the  engine  should  convert 
12  per  cent  of  the  heat  it  gets  as  gas. or  petroleum  into  indicated 
work.  But  the  diagram  is  imperfect  in  many  ways.  Using  the  mix- 
ture it  does,  the  diagram  should  show  a  maximum  temperature  of 
1600°  C.  at  least ;  in  reality  the  highest  temperature  is  only  840°  C. 
The  flame  is  entering  the  cylinder  at  an  actual  temperature  of 
1600°  C.  during  the  whole  period  of  admission,  but  the  convec- 
tion has  so  greatly  increased  by  the  mixing  effect  of  the  entering 
current  that  greatly  increased  cooling  results  ;  accordingly,  when  the 
gases  are  fully  admitted  and  the  inlet  valve  is  closed,  the  gases  have 
only  a  temperature  of  840°  C.  instead  of  1600°  C.  After  admis- 
sion ceases,  the  expansion  line  from  45  Ibs.  to  10  Ibs.  pressure  is  far 
above  the  adiabatic,  indeed  it  is  isothermal,  the  combustion  is 
proceeding  and  the  small  igniting  flame  also  is  helping  to  sustain 
the  temperature. 

It  is  therefore  quite  evident  that  the  loss  of  heat  is  much  greater 
than  that  occurring  during  explosion  in  equal  time.  The  correction 
of  the  theoretic  efficiency  indicated  by  the  author's  closed  vessel 
experiments  is  insufficient,  0*12  is  much  above  the  actual  effici- 
ency. Taking  the  heating  value  of  the  American  coal  gas  used 
in  Prof.  Thurston's  experiments  as  10,900  heat  units  per  unit  weight 

II 


1 62  The  Gas  Engine 

of  gas  burned,  and  one  pound  of  it  as  measuring  30  cubic  feet., 
then  as  the  engine  used  55  cubic  feet  per  IHP  per  hour,  it* 
efficiency  is  o'oyi;  that  is,  it  converts  7-1  per  cent,  of  the  heat  given 
to  it  into  work. 

This  is  a  poor  result  for  a  cycle  having  so  high  a  theoretic 
efficiency,  and  in  the  author's  experiments  with  petroleum  it  is 
even  worse. 

The  sp.  gravity  of  the  petroleum  was  0-85,  therefore  the  weight 
of  one  gallon  is  8*5  Ibs.  As  0-255  gallons  are  burned  per  indi- 
cated horse  power  per  hour,  this  amounts  to  8-5  x  0-255=  2 -i  6  Ibs. 
of  liquid  fuel  per  IHP  per  hour.  One  pound  gives  out  11,000 
heat  units,  and  for  one  horse  power  for  one  hour  1424  units  are 
required  ;  the  actual  indicated  efficiency  is  therefore 

1424  -'  1424 


=  o'o6  nearly  :  that  is,  6  per  cent,  of  the 
2-16x11000         23760 

whole  heat  given  to  the  engine  is  accounted  for  by  the  power 
developed  in  the  motor  cylinder. 

If  there  were  no  losses  of  heat  to  the  cylinder,  or  losses  by 
throttling  during  the  inlet  and  transfer  of  the  air  from  the  pump 
to  the  motor  or  loss  of  heat  from  the  reservoir  to  the  atmosphere, 
then  the  efficiency  of  this  type  of  engine  would  be  30  per  cent. 
These  losses  in  practice  reduce  it  to  6  per  cent.  The  cycle  is  a 
good  one,  and  under  other  circumstances  is  capable  of  better 
things,  but  it  is  quite  unsuitable  for  a  cold  cylinder  engine.  Cool- 
ing and  undue  resistance  are  the  main  causes  of  the  great  deficit. 

The  gases  entering  the  cylinder  as  flame,  in  passing  through 
the  inlet  chamber  expose  a  large  surface  to  the  action  of  the  water 
jacket ;  the  entering  currents  also  impinge  against  the  piston, 
causing  more  rapid  circulation  than  ordinary  convection.  Both 
causes  intensify  the  cooling  action  of  the  cylinder  walls.  In  the 
engine  tested  by  the  author  the  communicating  pipes  and  the 
motor  admission  valve  were  much  too  small  ;  a  considerable  loss 
of  pressure  resulted  ;  although  the  reservoir  pressure  was  60  Ibs., 
that  in  the  cylinder  never  exceeded  48  Ibs.  above  atmosphere, 
showing  a  loss  of  12  Ibs.  per  square  inch  from  undue  resistance. 
To  enable  this  engine  to  realise  the  advantages  of  its  theory  con- 
siderable modifications  in  its  arrangements  are  required.  Notwith- 
standing all  difficulties  it  has  done  much  useful  work,  not  the  least 


Gas  Engines  of  Different  Types  in  Practice         163 

notable  being  the  assistance  it  rendered  to  Prof.  Draper  during  his 
investigation  on  the  existence  of  non- metallic  bodies  in  the  sun's 
atmosphere.  He  used  a  Brayton  petroleum  engine  for  driving  his 
dynamo  machine,  and  he  stated  in  his  paper  that  its  ease  of  starting 
and  almost  absolute  steadiness  in  driving  were  of  the  greatest  ser- 
vice to  him.  In  steadiness  he  states  that  '  it  acted  like  an  instru- 
ment of  precision.' 


FIG.  48. — Simon  Engine. 

Si»ion  Engine.  —  Messrs.  Simon,  of  Nottingham,  introduced  the 
Brayton  engine  to  England  in  a  slightly  altered  form  as  a  gas 
engine.  In  addition  to  the  ordinary  arrangements  of  the  engine 
they  attempted  to  gain  increased  economy,  by  causing  the  waste 
heat  passing  into  the  water  jacket,  and  the  heat  of  the  exhaust 


164  The  Gas  Engine 

gases,  to  be  utilised  in  raising  steam.  They  would  undoubtedly 
have  increased  the  economy  of  the  engine  in  this  manner  had 
they  not  turned  the  steam  so  raised  into  the  motor  cylinder  along 
with  the  flame.  The  cooling  of  the  flame  which  was  serious 
enough  in  the  original  was  thus  made  worse,  and  but  slight  gain 
could  result,  the  loss  by  cooling  being  slightly  exceeded  by  the 
increase  of  volume  due  to  the  steam.  Fig.  48  is  an  external  view 
of  the  engine  as  exhibited  at  the  Paris  Exhibition  of  1878.  A  is 
the  motor,  B  the  pump,  and  c  the  added  boiler  ;  the  steam  was 
raised  in  it  and  the  water  jacket.  With  a  suitable  arrangement 
using  the  steam  in  a  separate  cylinder,  doubtless  6  per  cent,  might 


7  ins.  dia.  of  cylinder  ;  240  ft.  per  min.  piston  speed.     Scale  ^T  in. 
FIG.  49.  —  Diagram  from  Simon  Engine. 

be  added  to  the  indicated  efficiency  of  the  engine,  but  it  is  very 
questionable  if  the  increased  complexity  does  not  entirely  destroy 
any  advantage  gained ;  it  certainly  does  so  in  small  engines.  When 
very  large  engines  come  to  be  constructed  the  complexity  would 
not  be  so  great  and  it  would  be  well  worth  while  to  use  waste  heat 
in  steam  raising.  The  engine,  although  instructive,  did  not  suc- 
cessfully overcome  the  difficulties  which  caused  the  abandonment 
of  the  Brayton  as  a  gas  engine.  Fig.  49  is  a  diagram  from  the 
engine  which  forcibly  illustrates  the  effect  of  the  cooling. 


Gas  Engines  of  Different  Types  in  Practice         165 


TYPE  III. 

Engines  of  this  kind  resemble  those  just  discussed,  in  the  use 
of  compression  previous  to  ignition,  but  differ  from  them  in  ignit- 
ing at  constant  volume  instead  of  constant  pressure  ;  that  is,  the 
whole  volume  of  mixture  used  for  one  stroke  is  ignited  in  a  mass 
instead  of  in  successive  portions. 

The  whole  body  of  mixture  to  be  used  is  introduced  before 
any  portion  of  it  is  ignited  ;  in  the  previous  type  the  mixture  is 
ignited  as  it  enters  the  cylinder,  no  mixture  being  allowed  to  enter 
except  as  flame.  In  Type  III  the  ignition  occurs  while  the  volume 
is  constant ;  the  pressure  therefore  rises  ;  it  is  an  explosion  engine 
in  fact,  like  the  first  type,  but  with  a  more  intense  explosion  due 
to  the  use  of  mixture  at  a  pressure  exceeding  atmosphere. 

The  most  obvious  means  of  applying  the  method  is  that  sug- 
gested by  the  Lenoir  engine.  The  addition  of  a  pump  taking 
mixture  at  atmospheric  pressure,  compressing  it  into  a  reservoir 
from  which  it  passes  to  the  motor  cylinder  at  the  increased  pres- 
sure, seems  a  simple  matter.  The  igniting  arrangements  would 
act  as  in  the  original.  As  the  gases  are  under  pressure,  the  piston 
would  take  its  charge  into  the  cylinder  in  a  smaller  proportion  of 
the  forward  stroke,  and  so  more  of  the  motor  stroke  would  be 
available  for  useful  effect.  The  diagram  such  an  engine  should 
produce  is  seen  at  fig.  15,  p.  50 ;  the  shaded  part  is  the  available 
portion,  the  other  part  is  the  pump  diagram.  The  theoretic  effi- 
ciency of  such  an  engine  is  as  good  as  the  type  can  give.  The 
patent  list  shows  that  it  was  the  first  proposed  after  Lenoir.  Many 
such  engines  have  been  attempted  and  have  given  very  good  re- 
sults economically,  but  the  difficulties  of  detail  are  considerable, 
the  greatest  being  the  necessity  for  the  intermediate  reservoir. 
Million's  patent  1861  proposes  to  do  this,  the  present  author  also 
constructed  one  of  this  kind  in  1878,  and  later  one  was  made  by 
Mr.  Atkinson.  The  difficulties,  however,  are  too  great  to  allow  the 
success  of  small  motors  on  the  plan. 

Mr.  Otto,  the  first  to  succeed  with  the  free  piston  engine,  was 
also  the  first  to  succeed  in  adapting  compression  in  a  reliable 
form. 


1 66  The  Gas  Engine 

In  the  third  type  are  included  all  engines  having  the  following 
characteristics,  however  widely  the  mechanical  cycle  may  vary  : 

Engines  using  a  gaseous  explosive  mixture,  compressed  before 
ignition,  and  ignited  in  a  body,  so  that  the  pressure  increases  while 
the  volume  remains  constant.  The  power  is  obtained  by  expan- 
sion after  the  increase  of  pressure. 

Otto  Engine. — In  this  gas  engine,  the  first  to  combine  the 
compression  principle  with  a  simple  and  thoroughly  efficient  work- 
ing cycle,  the  difficulties  of  compression  are  overcome  in  a  strikingly 
original  manner.  To  the  engineer  accustomed  to  the  steam 
engine,  the  main  idea  seems  a  bold  and  indeed  a  retrograde  step. 
The  early  gas  engines  were  moulded  more  upon  the  steam  engine 
model  and  were  to  some  extent  double  acting.  The  Lenoir  and 
Hugon  both  received  two  impulses  for  every  revolution,  the 
Brayton  was  single  acting,  and  the  Otto  is  only  half  single  acting. 
The  steam  engine  in  its  advance  passed  from  single  to  double 
acting,  and  then  to  four  and  even  more  impulses  per  revolution. 
The  gas  engine  in  its  progress  has  in  this  respect  moved  backwards, 
beginning  with  double  action  and  then  going  back.  The  gain  of 
this  arrangement,  however,  has  completely  justified  the  retro- 
gression. 

In  external  appearance  the  engine  closely  resembles  a  modern 
high  pressure  steam  engine,  the  working  parts  of  which  are  of 
somewhat  excessive  strength  ;  its  motor  and  only  cylinder  is  hori- 
zontal and  open  ended  ;  in  it  works  a  long  trunk  piston,  the  front 
end  of  which  serves  as  a  guide  and  does  not  enter  the  cylinder 
proper  ;  the  connecting  rod  communicates  between  the  guide  and 
the  crank  shaft,  the  side  thrust  is  thus  kept  off  the  piston  and 
cylinder  proper,  which  become  hot.  The  crank  shaft  is  heavy  and 
the  fly-wheel  a  large  one  ;  considerable  energy  being  required  to 
take  the  piston  through  the  negative  part  of  the  cycle.  The 
cylinder  is  considerably  longer  than  the  piston  stroke,  so  that  the 
piston  when  full  in  leaves  a  considerable  space  into  which  it  does 
not  enter. 

Outside  the  cylinder,  running  across  it  at  the  end  of  the  space, 
works  a  large  slide  valve  ;  it  is  held  against  the  cylinder  face  by 
a  cover  plate  and  strong  spiral  springs  ;  it  is  driven  to  and  fro  by 
a  small  crank,  on  the  end  of  a  shaft  parallel  to  the  cylinder  axis, 


Gas  Engines  of  Different  Types  in  Practice         \  67 


f 

a 


1 68  The  Gas  Engine 

and  rotating  at  half  the  rate  of  the  crank  shaft,  from  which  it 
receives  its  motion  by  bevel  or  skew  gearing. 

An  exhaust  valve,  leading  into  the  space  by  a  port,  is  also 
actuated  at  suitable  times  from  the  secondary  shaft ;  so  are  the 
governing  and  oiling  gear. 

The  single  cylinder  serves  alternately  the  purposes  of  motor 
and  pump ;  during  the  first  forward  stroke  of  the  piston,  the  slide 
valve  is  in  such  position  that  gas  and  air  stream  into  the  cylinder 
from  the  beginning  to  the  end  of  the  stroke,  the  charge  mixing  as 
it  enters  with  whatever  gases  the  space  may  contain  ;  the  return 
stroke  then  compresses  the  uniform  mixture  into  the  space,  and 
when  the  piston  is  full  in,  the  pressure  has  increased  to  an  amount 
determined  by  the  relative  capacity  of  the  space.  Meantime  the 
slide  valve  has  moved  to  another  position,  first  closing  the  admission 
gas  and  air  ports,  to  permit  of  the  compression,  then  bringing  on  a 
cavity  in  the  valve  which  is  filled  with  flame,  when  the  compression 
is  completed.  The  compressed  charge  therefore  ignites  and  the 
pressure  rises  so  rapidly  that  maximum  is  attained  before  the  piston 
has  moved  appreciably  on  its  forward  stroke  (second  stroke)  ; 
the  piston  is  thus  under  the  highest  pressure  at  the  beginning  of 
its  stroke  and  the  whole  stroke  is  available  for  the  expansion. 

This  is  the  motive  stroke.  At  the  end  of  it,  the  exhaust  valve 
opens  and  the  return  stroke  is  occupied  in  driving  out  the  burned 
gases,  except  that  portion  remaining  in  the  space  which  cannot  be 
entered  by  the  piston.  These  operations  form  a  complete  cycle, 
and  the  piston  is  again  in  the  position  to  take  in  the  charge  re- 
quired for  the  next  impulse. 

The  cycle  requires  two  complete  revolutions,  or  four  single 
strokes. 

First  out  stroke.  Charging  cylinder  with  gas  and  air. 

„     in        „  Compressing  the  charge  into  the  space. 

Second  out  stroke.  Explosion  impelling  piston. 

„         in        „  Discharging  burned  gases  into  atmosphere. 

The  regulation  of  the  speed  of  the  engine  is  accomplished  by 
a  centrifugal  governor,  which  is  arranged  to  close  a  gas  supply 
valve  whenever  the  speed  increases.  An  explosion  is  thereby 
missed,  and  the  engine  goes  through  its  cycle  as  usual,  but  as  no 


Gas  Engines  of  Different  Types  in  Practice         169 


170  The  Gas  Engine 

gas  is  mixed  with  the  air,  there  is  no  explosion  when  the  flame 
enters,  the  compressed  air  merely  expanding,  giving  back  to  the 
piston  the  energy  taken  during  compression. 

When  running  without  load,  8  or  even  more  revolutions  may  be 
made  between  the  impulses,  at  full  load  2  revolutions  are  made  per 
impulse.  Notwithstanding  this  irregularity  the  fly-wheel  is  so 
large  that  no  variation  observab'e  by  the  eye  can  be  seen  while 
watching  the  engine. 

Fig.  50  is  an  external  elevation  of  an  Otto  engine. 

Fig.  51  is  a  sectional  plan,  and  fig.  52  an  end  elevation  showing 
exhaust  valve  lever.  A  is  the  water-jacketed  cylinder,  B  the  piston 
shown  full  in,  c  is  the  compression  space  or  cartridge  space  as  it 
is  called  by  Million;  i  the  admission  and  ignition  port,  communi- 
cating alternately  with  the  gas  and  air  admission  port  K,  and  the 
flame  port  L  in  the  slide  M  ;  N  is  the  cover  holding  the  slide  to  the 
cylinder  face  and  carrying  in  it  the  external  flame  for  lighting  he 
movable  one  in  flame  port  L.  The  exhaust  valve  is  of  the  conici  1 
seated  lift  type  and  is  seen  at  o;  it  is  driven  from  the  shaft  P  by 
the  cam  Q  and  the  lever  R.  The  other  details  are  clearly  shown 
upon  the  drawing.  The  ignition  valve  and  governing  arrangement 
will  be  described  in  a  subsequent  chapter  ;  here  it  is  sufficient 
to  state  that  the  governor  withdraws  a  cam  actuating  the  gas 
valve  s,  fig.  52,  and  so  prevents  it  opening  when  the  piston  is 
taking  in  air.  When  open,  the  gas  passes  the  valve,  then  through 
a  row  of  holes  in  the  valve  port  K,  streaming  into  the  air  and  mix- 
ing thoroughly  with  it  as  it  enters  the  cylinder.  To  start  the 
engine,  the  flame  at  T  is  lighted  ;  the  cock  commanding  the  internal 
flame  being  properly  adjusted,  and  the  gas  turned  on,  a  couple  of 
turns  at  the  fly-wheel  should  cause  ignition  and  set  the  engine 
in  motion.  The  larger  engines  are  provided  with  a  second  cam, 
which  keeps  the  exhaust  valve  open  during  half  of  the  compression 
stroke  and  so  diminishes  the  work  required  to  turn  round  the  engine 
by  hand.  When  the  engine  is  started  the  wheel  upon  the  lever  is 
shifted  to  the  normal  cam  and  the  compression  then  returns  to  its 
usual  intensity. 

Diagrams  and  Gas  Consumption. — Dr.  Slaby,  of  Berlin,  has 
made  a  very  careful  trial  of  a  four-horse  power  Otto  engine  at 
Mr.  Otto's  works,  Dtutz,  in  August  1881. 


Gas  Engines  of  Different  Types  in  Practice         171 


FIG.  52. -Otto  Engine  (End  elevation). 


172  The  Gas  Engine 

The  dimensions  of  the  engine  are  : 

Diameter  of  cylinder 171 '9  millimetres. 

Stroke 340  millimetres. 

Compression  space  ......  4770  cb.  centimetres. 

Volume  displaced  by  piston     ....  7888  cb.  centimetres. 

The  compression  space  is  therefore  o'6  of  the  volume  displaced  by 
the  piston.     The  results  are  briefly  as  follows  : 

Average  revolutions  during  test        .         .         .  1567  per  minute. 

Power  indicated  in  cylinder      .         .         .         .  5  "04  horse. 

Power  by  dynamometer   .         .         .         .         •  4'4  horse. 

Gas  consumed  in  one  hour       ....  142-67  cb.  ft. 

Gas  con  .um  3d  in  one  hour  by  igniting  flames  .  275  cb.  ft. 

Gas  consumption  per  IHP  per  hour         .         .  28*3  cb.  ft. 

Gas  consumption  per  effective  HP  hour  .         .  32^4  cb.  ft. 

The  composition  of  the  gas  used  at  the  Gasmotoren-Fabrik, 
Deutz,  is  given  as — 

Volumes. 
Marsh  gas,  CH4        .         .         .         .         .         .         .         .         .     34-4 

Ethylene,  C2H4 3'5 

Hydrogen,  H    ..........     56^9 

Carbonic  oxide,  CO 5 '2 


and  i  cubic  metre  of  it  weighs  0-404  kilograms.  One  pound 
weight  of  it  therefore  measures  39-6  cubic  feet.  Deducting  the 
latent  heat  of  steam  produced,  i  pound  weight  evolves  heat  enough 
to  raise  12,094  Ibs.  of  water,  through  one  degree  Centigrade.  It 
evolves  1 2, 094  heat  units.  From  this  value,  and  the  experimental 
determination  of  the  heat  leaving  the  engine  by  way  of  the  water 
jacket,  Dr.  Slaby  calculates  the  disposition  of  100  heat  units  given 
to  the  engine  as  follows  : 

Work  indicated  in  c\  Under        .......  i6'o 

Heat  lost  to  cylinder  walls         .         .         .         .         .         .  =;i'o 

Heat  carried  away  by  exhaust  .         .         .         .         .         .         .  31-0 

Heat  lost  from  engine  by  conduction  and  radiation  .         .         .  2'o 

lOO'O 

The  actual  indicated  efficiency  of  the  engine  is  therefore  16 
per  cent,  or  o'i6. 

The  temperature  of  the  gases  expelled  during  the  exhaust 
stroke  was  determined  by  carefully  protecting  the  exhaust  pipe 


Gas  Engines  of  Different  Types  in  Practice         173 

from  loss  of  heat  by  non-conducting  material,  and  then  seeing 
whether  zinc,  or  antimony  would  melt  in  it.  Zinc  melted  but 
antimony  did  not ;  as  the  melting  point  of  zinc  is  423°  C,  and  the 
antimony  melting  point  is  432°  C.,  the  temperature  of  the  exhaust 
gases  is  given  with  great  accuracy  as  between  these  two  tempera- 
tures. The  average  composition  of  the  mixture  is  given  as  i  vol. 
coal  gas  to  1373  vols.  of  air  and  other  gases.  Here  Dr.  Slaby  is 
plainly  in  error,  as  his  own  figures  conclusively  show.  The  volume 
of  coal  gas  taken  into  the  engine  at  each  stroke  as  measured 
by  the  gas  meter  is  given  as  859  cubic  centimetres,  the  total 
volume  swept  by  the  piston  of  the  engine  per  stroke  is  7888  cubic 
centimetres,  the  volume  of  the  compression  space  4770  cubic 
centimetres.  Now  if  the  gas  be  introduced  into  the  cylinder 
while  it  is  filled  completely,  space  included,  with  cold  gases,  at 
the  same  temperature  as  the  gases  when  measured  by  the  meter, 
this  figure  is  correct  enough.  But  the  gases  are  not  so  introduced, 
the  space  is  already  filled  with  exhaust  gases  at  a  temperature  of 
about  400°  C.  by  Dr.  Slaby's  own  determination  ;  this  volume 
must  therefore  be  calculated  to  atmospheric  temperature  before 
an  approach  to  the  true  ratio  can  be  obtained.  Taking  atmo- 
spheric temperature  at  17°  C.,  then  4770  cubic  centimetres  of 
burned  gases  at  400°  C.  becomes  reduced  to  2055  cubic  centi- 
metres at  17°  C. ;  that  is,  the  total  charge  will  consist  of  859  cubic 
centimetres  of  coal  gas,  7029  cubic  centimetres  of  air,  and  2055 
cubic  centimetres  of  burned  gases  from  the  previous  explosion. 
The  ratio  is 

coal  gas  859  i 

air  and  burned  gases      7029  +  2055  ~~  10-5 

The  composition  of  the  charge  is  more  correctly  represented 
as  i  vol.  of  gas  to  10-5  vols.  of  air  and  other  gases.  Even  here, 
however,  the  dilution  is  overstated,  as  it  is  assumed  that  the 
piston  has  taken  in  the  charge  at  full  atmospheric  temperature 
and  pressure.  But  there  is  some  throttling  in  passing  through  the 
admission  valve  and  port,  and  also  some  heating  of  the  air  by 
striking  the  piston  and  cylinder  walls.  Professor  Thurston,  in 
experiments  to  be  described  later  on,  proves  this  to  be  the  case, 
and  shows  that  the  charge  is  even  stronger  than  has  been 
calculated. 


1 74  The  Gas  Engine 

It  has  been  already  proved  that  in  this  type  of  engine,  ex- 
panding after  compression  and  explosion  to  the  same  volume  as 
existed  before  compression,  the  theoretic  efficiency  is  independent 
of  the  temperature  of  the  explosion  or  the  temperature  existing 
before  compression,  and  depends  only  upon  the  volume  before  and 
after  complete  compression.  As  the  ratio  of  compression  space 
to  volume  swept  by  the  piston  is  o-6  to  i,  the  volume  before 
compression  is  i-6,  volume  after  compression  o-6. 

(-1  \  y- « 
~'} 

and  TC  is  the  compression  volume,  and  r0  the  volume  before  com- 

/o-6\  "4°8  /    i    \  '4o8  • 

pression  :  in  this  case  E  =  i  -  or  i  —  I 

\i  6/  \2'66/ 

here  E  =  0-33. 

That  is,  if  all  the  heat  were  given  to  the  engine  at  the  moment 
of  complete  explosion  at  the  beginning  of  the  stroke,  and  no  heat 
were  lost  to  the  cylinder  during  the  expansion  to  the  original 
volume,  then  33  per  cent,  of  that  heat  would  be  converted  into 
indicated  work.  But  the  author's  explosion  experiments  give  the 
factor  necessary  for  correcting  this  theoretical  number  (p.  113). 
Taking  the  mixture  of  i  gas  to  10  vols.  air  as  nearest,  the 
efficiency  of  the  gas  in  it  is  0-46  ;  that  is,  during  the  time  of  the 
forward  stroke,  taken  as  0-2  sec.,  i  vol.  of  gas  is  required  to  pro- 
duce and  keep  up  a  pressure  which  0-46  vol.  would  suffice  for  if 
it  was  all  applied  to  heating  and  no  loss  by  cooling. 

The  actual  indicated  efficiency  of  the  engine  using  this 
mixture  and  this  expansion  and  compression  should  be  0*33  x 
0-46=0-152  nearly.  That  is,  the  engine  should  convert  15-2  per 
cent,  of  the  heat  given  to  it  into  work.  Dr.  Slaby's  number,  found 
by  experiment,  is  16  per  cent.  The  numbers  are  exceedingly 
close. 

The  mechanical  efficiency  of  the  engine  is  high,  the  ratio  of 
dynamcmetric  to  indicated  power  being  87  to  100,  and  the  friction 
of  the  engine  only  0^64  horse 


Gas  Engines  of  Different  Types  in  Practice         175 


PROFESSOR  THURSTON'S  EXPERIMENTS  ON  A  6  HP  OTTO 
ENGINE. 

Dr.  Slaby's  experiments  are  exceedingly  complete,  but  Pro- 
fessor Thurston  in  America  has  made  even  more  extended 
measurements. 

Messrs.  Brooks  and  Steward  made  the  trials  under  the  direc- 
tion of  Professor  Thurston,  at  the  Stevens  Institute  of  Technology, 
Hoboken.  The  dimensions  of  the  engine  are  as  follows  : 

Diameter  of  cylinder    .         .         .         .         .         .         .  8 '5  ins. 

Stroke 14  ins. 

Capacity  of  compression  space  38  per  cent,  of  total  cylinder 
volume. 

Not  only  was  the  gas  entering  the  engine  measured,  but  at  the 
same  time  the  air  required  was  measured  through  a  300  light 
meter.  So  far  as  the  author  is  aware,  this  is  the  only  set  of 
experiments  in  which  this  was  done  ;  it  is  by  far  the  most  accurate 
way  of  getting  the  true  proportions  of  the  explosive  mixture. 

The  temperature  of  the  exhaust  was  measured  by  a  pyrometer, 
and  the  power  determined,  both  by  indicator  and  dynamometer  ; 
at  the  same  time  the  heat  passing  into  the  walls  of  the  cylinder 
was  determined  by  measuring  the  water  heated  and  estimating 
the  loss  by  radiation  and  conduction. 

The  total  number  of  revolutions  during  the  various  tests  were 
taken  by  a  counter.  Many  trials  were  made  under  varying  con- 
ditions of  load  and  mixture  used  The  following  is  the  best  full- 
power  trial,  giving  the  most  economical  results  : 

Average  revolutions  during  test     .....  158  per  minute. 

Power  indicated  in  cylinder 9 "6  hors?. 

Power  by  dynamometer        ......         8-r  horse. 

Gas  consumed  in  one  hour 235  cb.  rt. 

Gas  consumpi ion  per  IHP  p  r  hour     ....  24^5  cb.  ft. 

Gas  consumption  per  effective  HP  per  hour          .         .  29*1  cb.  ft. 

An  analysis  of  the  gas  used  during  the  trials  made  by  Thomas 
B.  Stillman,  Ph.D.,  is  as  follows  : 


176  The  Gas  Engine 

Hydrogen,  H 39'5 

Marsh  gas,  CH4 37'3 

Nitrogen,  N         .         .         .         .         .         •         •                  .  8'2 

Heavy  hydrocarbons,  C2H6,  &c .        .  6-6 

Carbonic  oxide,  CO 4 '3 

Oxygen,  O J'4 

Water  vapour  and  impurities  (H2O,  CO>,  H2S)   .        .        .27 


One  cubic  metre  of  this  gas  weighs  0*606  kilograms.  One 
pound  weight  of  it  therefore  measures  26^43  cubic  feet.  One 
pound  when  completely  burned  evolves  heat  enough  to  raise  9070 
Ibs.  water  through  i°  C. 

The  air  necessary  to  supply  just  enough  oxygen  for  the 
complete  combustioaof  i  vol.  of  this  gas  is  5^94  vols. 

From  these  values  and  experiments  upon  temperature  of  the 
exhaust  gases,  Professor  Thurston  estimates  the  disposition  of  100 
heat  units  by  the  engine  as  follows : 

Work  indicated  in  cylinder .         .         .         .         .         .         .17*0 

Heat  lost  to  cylinder  walls    .         .         .         .        .         .         .     52^0 

Heat  carried  away  by  exhaust  gases 15-5 

Heat  lost  from  engine  by  conduction  and  radiation     .        .     15-5 

100 'O 

The  actual  indicated  efficiency  is  therefore  1 7  per  cent. 

The  number  showing  the  proportion  of  heat  passing  into  the 
water  jacket  is  also  very  nearly  Slaby's,  but  the  amount  expelled 
with  the  exhaust  is  much  understated.  The  amount  lost  by  radi- 
ation is  overstated. 

The  temperature  of  the  exhaust  gases,  as  determined  by  a 
pyrometer  placed  in  the  exhaust  pipe,  varies  in  the  experiments  at 
full  load  from  399°  C.  to  432°  C.,  thus  practically  coinciding  with 
Slaby.  The  ratio  of  air  to  gas  was  found,  by  actual  measurement 
of  both,  to  be  about  7  to  i  when  the  engine  was  working  most 
economically.  Although  with  better  gas  the  ratio  would  be 
slightly  increased,  yet  it  could  not  equal  that  usually  given  for  the 
Otto  engine,  10  to  i  or  thereabouts. 

The  ratio  is  common'y  obuincd  from  a  measurement  of  the 
gas  consumption  alone,  the  air  being  reckoned  as  the  volume  of 
the  piston  displacement,  less  the  measured  amount  of  gas.  This 
is  not  an  accurate  method,  for  the  reason  already  stated. 


Gas  Engines  of  Different  Types  in  Practice.        177 


If  the  mixture  filling  the  cylinder  mingles  with  the  burned 
gases  filling  the  compression  space,  then  the  average  composition 
ot  the  charge  is  i  voL  coal  gas  to  9*1  vols.  of  other  gases. 


M 

c 
W 
o 

1    5 

5 


Q 


178  The  Gas  Engine 

Fig.  53  is  a  fair  sample  of  the  diagrams  obtained  during  Pro- 
fessor Thurston's  tests  while  the  engine  was  giving  full  power. 
The  piston  while  moving  from  the  point  i  to  the  point  2  takes 
in  the  charge  ;  the  pressure  in  the  cylinder  falls  below  atmo- 
sohere  as  the  piston  approaches  the  end  of  its  stroke.  This  is 
due  to  the  resistance  of  the  valve  port  to  entering  air  and 
gas.  The  piston  returns  from  2  to  5  (ist  in-stroke)  compress- 
ing the  charge,  the  pressure  increasing  to  atmosphere  at  the 
point  3,  the  compression  being  complete  at  the  point  5  ;  the 
ignition  then  occurs,  and  the  pressure  and  temperature  rapidly 
rises  as  the  explosion  progresses ;  the  temperature  does  not 
attain  its  maximum  till  the  piston  has  moved  forward  a  little  and 
has  reached  the  point  6.  From  that  point  to  7,  when  the  ex- 
haust vaive  opens,  the  expanding  line  is  as  nearly  as  possible 
adiabatic.  The  temperatures  are  marked  at  each  point  of  the 
diagram.  The  return  stroke  from  2  to  i  discharges  the  products 
of  combustion.  This  is  the  second  in-stroke,  completing  the 
cycle  and  leaving  the  engine  in  position  to  again  take  in  the 
charge. 

The  diagram  shows  the  whole  changes  occurring  during  two 
complete  revolutions  of  the  machine  while  fully  loaded.  Fig.  54 
shows  what  occurs  when  the  governor  acts,  when  the  engine  is  at 
less  than  full  load.  The  smaller  diagram,  B,  is  the  normal  one, 
and  the  larger,  A,  the  intermittent  one  ;  the  gas  has  been  com- 
pletely cut  off  for  several  strokes,  and  so  the  hot  burned  gases  in 
the  compression  space  have  been  completely  discharged  and  re- 
placed by  pure  air  at  a  temperature  not  far  removed  from  atmo- 
spheric ;  the  explosion  then  causes  a  higher  pressure  by  nearly  half 
an  atmosphere,  although  the  maximum  temperature  is  less  than  in 
the  usual  case. 

The  temperature  of  the  charge  before  explosion  being  less,  a 
smaller  increase  is  required  to  produce  a  given  increase  of  pressure. 
Professor  Thurston  calculates  that  the  heat  accounted  for  by  the 
diagram  is  60  per  cent,  of  the  total  heat  supplied  to  the  engine ; 
the  deficiency  he  attributes  to  the  phenomena  of  dissociation, 
which  prevents  the  complete  evolution  of  the  heat  at  the  highest 
temperature,  but  permits  further  combustion  when  the  temperature 
falls.  The  amount  of  gas  required  to  run  at  full  speed,  166  revo- 


Gas  Engines  of  Different  Types  in  Practice         179 

lutions  per  minute  without  any  load,  was  found  to  be  from  50  to 
70  cubic  feet  per  hour. 


W 


.3 
6 


of  Otto  Engine.— The  experiments  of  Dr.  Slaby  and 
Professor  Thurston  upon  the  Otto  engine  are  by  far  the  most 
complete  which  have  yet  been  made  to  the  author's  knowledge. 


1 80  The  Gas  Engine 

Some  tests  given  in  Schottler,  however,  will  be  quoted.  A  four 
HP  engine  was  found  to  consume  as  a  best  result  32-4  cubic 
feet  of  gas  per  brake  HP  per  hour  in  Altona,  giving  at  the  time 
3-96  HP  on  the  dynamometer.  Another  consumed  337  cubic 
feet  per  brake  HP  per  hour  in  Hanover,  giving  4-95  HP  on 
the  dynamometer;  to  drive  the  last  engine  at  160  revolutions  per 
minute  without  load  required  41-3  to  43*4  cubic  feet  of  Hanover 
gas. 

A  two-horse  engine,  tested  by  Erauer  and  Slaby,  Berlin,  gave 
2-28  brake  HP,  using  35-3  cubic  feet  per  biake  HP  per  hour. 

In  this  country  the  coal  gas  in  common  use  is  of  higher  heat- 
ing value  than  that  used  on  the  Continent  and  in  America ;  ac- 
cordingly the  gas  required  per  HP  is  less,  but  the  efficiency  is 
almost  identical. 

Experiments  made  upon  an  8  HP  Otto  engine  by  the  Philo- 
sophical Society  of  Glasgow  in  1880,  showed  a  consumption  of 
22  cubic  feet  of  Glasgow  gas  per  indicated  HP,  giving  9  HP  upon 
the  dynamometer,  and  28  cubic  feet  per  dynomemetric  horse. 

Experiments  made  at  the  Crystal  Palace  Electrical  Exhibition, 
in  1881,  with  a  12  HP  engine  gave  a  maximum  brake  power  of 
i8'3  HP,  with  a  gas  consumption  of  237  cubic  feet  per  IHP,  and 
29'!  cubic  feet  per  brake  HP.  With  a  two-horse  engine,  2*87 
brake  horse  was  obtained  upon  33-4  cubic  feet  per  horse  hour, 
and  27-9  cubic  feet  per  indicated  horse  hour. 

The  consumption  running  without  load  does  not  seem  to  have 
been  taken  in  these  tests. 

The  author  has  taken  the  consumption  of  a  two-horse  engine 
running  without  load  in  London,  at  160  revolutions  per  minute,  as 
32  cubic  feet  per  hour,  and  a  3-5  horse  engine  without  load  at 
1 66  revolutions  per  minute  as  43  cubic  feet  per  hour. 

The  Messrs.  Crossley  give  the  following  as  the  results  with 
their  new  Otto  twin  engine  rated  at  12  HP  : 


Power  by  dynamometer 
Power  indicated  in  cylinders 
Gas  consumption  per  indicated  HP 
Gas  consumption  per  effective  HP 
Tcial  consumption  at  full  power  . 


23  horse. 

28  horse. 

20  cb.  ft.  per  hour. 

24-3  cb.  ft.  per  hour. 

560  cb.  ft.  per  hour. 


Total  consumption  when  running  without  load  at 

160  revs,  per  minute .     100  cb.  ft.  per  hour. 


Gas  Engines  of  Different  Types  in  Practice         181 


HB^S 

0  S 1 1  3 ,1 
floa 


o 


w 


1 82  The  Gas  Engine 

These  results  are  obtained  using  Manchester  gas. 
Mr.  G.  H.  Garrett  has  made  a  trial  with  an  8  HP  Otto  engine 
in  Glasgow,  the  diagram  and  particulars  of  which  are  given  on 

fig-  55- 

Summary  of  Experiments. — From  these  numerous  and  careful 

experiments,  conducted  quite  independently  of  each  other  by 
many  observers,  it  may  be  taken  as  abundantly  established  that 
the  Otto  engine  is  a  great  advance  in  economy  and  certainty  of 
action  upon  any  gas  engine  preceding  it.  On  the  Continent  and 
in  America  the  consumption  per  horse  power  hour  is,  on  the 
whole,  greater  than  in  Britain  ;  this  is  due  not  to  any  appreciable 
difference  in  the  efficiency  of  the  engines  made  here,  but  to  the 
better  gas  common  in  this  country. 

Calculations  of  the  efficiency  attained  in  someof  the  later  engines 
in  England,  show  that  as  much  as  18  per  cent,  of  the  heat  is  con- 
verted into  work  as  shown  by  the  indicator.  Dr.  Slaby's  value  is 
1 6  per  cent,  and  Professor  Thurston's  17  per  cent.  All  observers 
agree  that  the  heat  liberated  at  the  moment  of  completed  explosion, 
that  is,  of  highest  temperature,  is  roughly  one-half  of  the  total  heat 
present  as  coal  gas,  the  remaining  half  being  evolved  during  the 
expansion  period.  Professor  Thurston  gives  the  heat  of  the  ex- 
plosion as  60  per  cent,  of  the  total  heat  present,  Dr.  Slaby  as  55 
per  cent  The  author's  experiments  upon  the  heat  evolved  by 
the  explosion  of  different  mixtures  of  gas  and  air,  show  heat 
accounted  for  by  the  explosion  as  ranging  from  50  to  60  per  cent., 
agreeing  with  the  determinations  of  Bunsen,  Him,  Mallard  and 
Le  Chatelier,  and  Berthelot  and  Vieille.  It  may  therefore  be 
considered  as  absolutely  proved  that  this  suppression  of  heat  at 
explosion,  and  its  evolution  during  expansion,  is  a  phenomenon 
inherent  in  every  explosive  mixture,  however  made — a  thing,  in 
fact,  from  which  there  is  no  escape.  In  whatever  way  an  engine 
be  made,  if  it  explodes  or  burns  a  mixture  of  any  inflammable 
gas  with  any  mixture  of  gases  containing  oxygen,  then  this  slow 
combustion  or,  as  the  Germans  have  it,  nachbrennen  (after-burn- 
ing) is  unavoidably  occasioned.  Knowing  this,  and  knowing  of 
Him,  Bunsen,  and  Mallard  and  Le  Chatelier's  work  long  prece- 
dent to  Dr.  Slaby's  report,  it  is  surprising  to  find  so  able  and 
learned  a  scientist  quoted  as  stating  that  in  the  Lenoir  engine 


Gas  Engines  of  Different  Types  in  Practice         183 

the  whole  heat  was  evolved  at  the  moment  of  complete  explosion. 
In  the  Lenoir,  as  in  every  other  gas  engine  which  has  ever 
been  constructed,  not  more  than  one-half  of  the  whole  heat  of 
the  gas  present  is  then  evolved,  the  remaining  heat  being  evolved 
on  the  expanding  stroke. 

Schottler  falls  into  the  same  error,  and,  although  mentioning 
Wedding's  statement  of  Bunsen's  law  of  dissociation,  shows  that 
he  rejects  it  when  he  assumes  that  the  whole  heat  is  evolved.  A 
very  cursory  examination  of  the  Lenoir  diagram  would  at  once 
prove  to  Prof.  Schottler  that  Lenoir  did  not  succeed  in  so 
escaping  the  laws  of  nature ;  had  he  done  so,  there  would  have 
been  no  necessity  for  our  modern  improvements. 

The  consumption  of  continental  gas  may  be  taken  as  varying 
between  32  cb.  ft.  and  35  cb.  ft.  per  effective  HP  per  hour,  and 
about  28  cb.  ft.  per  IHP  per  hour. 

In  Britain  it  may  be  taken  as  ranging  from  24  cb.  ft.  pei 
effective  HP  to  33  cb.  ft.,  and  20  to  24  cb.  ft.  per  IHP  per  hour, 
depending  upon  the  quality  of  gas  used  and  on  the  dimensions 
of  the  engine  tested.  Other  things  being  equal,  better  results 
are  obtained  with  large  engines.  The  theoretic  efficiency  is  con- 
stant for  both  large  and  small  engines  where  the  same  compression 
is  in  use,  but  the  loss  of  heat  from  the  explosion  to  the  sides  of 
the  cylinder  is  less  in  the  large  engines,  due  to  the  diminished 
surface  exposed  in  proportion  to  the  volume  used.  The  effect  is 
to  increase  the  efficiency  of  the  gas  in  the  mixture  used,  a 
smaller  quantity  being  necessary  to  make  up  for  the  loss  of 
heat. 

The  indicator  diagrams  prove  the  very  efficient  nature  of  the 
Otto  cycle.  The  great  simplicity  attained  by  the  alternate  use  of 
the  cylinder  as  pump  and  motor  diminishes  the  number  of  valves 
necessary,  and  secures  the  minimum  resistance  to  the  entering 
gases,  while  entirely  preventing  any  loss  due  to  ports,  in  trans- 
ferring the  gases  from  one  cylinder  to  another.  The  carrying  out 
of  the  cycle  is  mechanically  almost  perfect,  no  work  being  spent 
which  is  not  included  in  the  theory.  Again,  the  piston  is  full  in 
at  the  moment  of  ignition  and  is  almost  at  rest  ;  the  heat,  pro- 
ducing maximum  temperature,  is  therefore  added  at  nearly 
constant  volume.  The  highest  pressure  which  the  gas  present  is 


184  The  Gas  Engine 

capable  of  producing  is  therefore  attained  at  the  beginning  of  the 
stroke  simultaneously  with  the  highest  temperature  ;  the  succeed- 
ing expansion  is  then  very  rapid,  and  so  no  unnecessary  waste  of 
heat  occurs,  the  temperature  being  rapidly  depressed  by  work 
being  done.  The  united  effect  of  all  the  arrangements  is  seen  in 
a  diagram  which  is  almost  theoretically  perfect  ;  the  only  de- 
duction from  theory  is  due  to  the  unfortunate  property  of  explo- 
sive mixtures  of  continued  combustion  after  explosion.  And 
this  reduces  the  theoretic  efficiency  to  one-half  in  practice.  The 
theoretic  efficiency  of  all  Otto  engines,  of  whatever  dimension,  is 
0-33,  as  the  compression  space  in  all  cases  bears  nearly  the  ratio 
of  o-6  to  i'o  when  compared  with  the  cylinder  volume  which  is 
swept  by  the  piston.  The  actual  indicated  efficiency  is  very 
nearly  one-half  of  that  number. 

If  combustion  by  any  means  could  be  made  complete  at  the 
highest  temperature  and  pressure  at  the  beginning  of  the  stroke, 
instead  of  continuing  as  it  does  well  into  the  expansion  stroke, 
then  greatly  increased  economy  would  result,  and  in  large  engines 
theory  might  be  very  nearly  approached. 

This  point  will  receive  further  discussion  later  on. 

Clerk  Engine. — Otto's  method  is  probably  the  readiest  and 
easiest  solution  of  the  problem  of  attaining  in  a  practicable 
manner  the  advantages  of  compression  ;  in  some  points,  however, 
the  advantages  are  accompanied  with  compensating  disadvantages. 

Only  one  impulse  for  every  two  revolutions  is  obtained  ;  the 
engine  is  therefore  stronger  and  heavier  than  need  be  if  impulse 
every  revolution  were  possible.  It  is  also  more  irregular  in  its 
action  than  more  frequent  impulses  would  give. 

The  Clerk  engine  was  invented  by  the  author  with  the  view  of 
obtaining  impulse  at  every  revolution,  while  getting  at  the  same 
time  the  economy  due  to  compression. 

At  first  blush  it  seems  a  very  simple  matter  to  make  a  com- 
pression gas  engine  to  give  an  impulse  for  every  revolution  ;  this 
was  the  author's  opinion  when  he  commenced  work  for  the  first 
time  upon  gas  engines  using  compression  in  October  1876.  Since 
then  he  has  had  occasion  to  modify  the  opinion  :  the  difficulties 
are  very  great  ;  any  engineer  who  doubts  this  will  speedily  be 
convinced  upon  making  the  attempt. 


Gas  Engines  of  Different  Types  in  Practice         185 

It  was  not  till  the  end  of  1880  that  the  author  succeeded  in 
producing  the  present  Clerk  engine  ;  before  that  time  he  had 
several  experimental  engines  under  trial,  one  of  which  was  ex- 
hibited at  the  Royal  Agricultural  Society's  show  at  Kilburn  in 
July  1879.  This  engine  was  identical  with  the  Lenoir  in 
idea,  but  with  separate  compression  and  a  novel  system  of 
ignition. 

The  Clerk  engine  at  present  in  the  market  was  the  fiist  to 
succeed  in  introducing»compression  of  this  type,  combined  with 


FIG.  56.— The  Clerk  Gas  Engine. 

ignition  at  every  revolution  ;  many  attempts  had  previously  been 
made  by  other  inventors,  including  Mr.  Otto  and  the  Messrs. 
Crossley,  but  all  had  failed  in  producing  a  marketable  engine. 
It  is  only  recently  that  the  Messrs.  Crossley  have  made  the  Otto 
engine  in  its  twin  form  and  so  succeeded  in  getting  impulse  at 
every  turn. 

In  the  Clerk  engine  the  whole  cycle  is  completed  in  one  re- 
volution, and  an  impulse  given  to  the  crank  on  every  forward 
stroke  of  the  piston,  when  working  at  full  power. 

The  engine  contains  two  cylinders,  one  for  producing  power, 


1 86  The  Gas  Engine 

the  other  for  taking  in  the  combustible  charge  and  transferring  it 
to  the  power  cylinder.  At  the  end  of  the  motor  cylinder  is  left 
a  compression  space  of  a  conical  shape,  and  communicating  with 
the  charging  or  displacing  cylinder  by  a  large  automatic  lift-valve 
opening  into  the  space  ;  at  the  other  end  of  the  cylinder  are  placed 
V-shaped  ports  opening  to  the  atmosphere  by  the  exhaust  pipe  ; 
the  motor  piston,  when  near  its  outer  limit,  overruns  these  ports 
and  allows  the  cylinder  to  discharge.  The  pistons  are  connected 
in  the  usual  manner  by  connecting  rods,  the  motor  to  the  main  crank 
of  the  engine,  the  displacer  to  a  crank  pin  in  one  of  the  arms  of 
the  fly-wheel;  the  displacer  crank  is  in  advance  of  the  motor  crank, 
in  the  direction  of  motion  of  the  engine,  by  a  right  angle.  The 
displacer  piston  on  its  forward  movement  takes  in  its  charge  of 
gas  and  air,  and  has  returned  a  fraction  of  its  stroke  when  the 
motor  piston  uncovers  the  exhaust  ports.  While  crossing  the  centre, 
opening  and  shutting  these  ports  the  displacer  piston  has  moved 
in  almost  to  the  end  of  its  cylinder,  discharging  its  contents 
into  the  space  and  forcing  out  at  the  exhaust  ports  the  products  of 
the  previous  ignition.  The  proportions  of  the  two  cylinders  are  so 
arranged  that  the  exhaust  is  as  completely  as  possible  expelled, 
and  replaced  by  cool  explosive  mixture,  which  thoroughly  mixes 
with  any  exhaust  remaining,  cooling  it  also  to  a  considerable 
extent.  Care  must  be  taken  in  the  arrangement  of  the  parts 
that  an  excessive  volume  is  not  sent  from  the  displacer, 
otherwise  it  may  reach  the  exhaust  ports  and  gas  discharge 
unburned. 

The  return  stroke  of  the  motor  piston  now  compresses  the 
mixed  gases,  and  when  at  the  extreme  end,  the  igniting  valve  fires 
the  mixture,  the  piston  moves  forward  under  the  pressure  thereby 
produced,  till  the  opening  of  the  exhaust  ports  causes  discharge 
and  replacement  as  before.  In  this  way  an  impulse  is  given  at 
every  revolution,  and  the  motive  power  applied  to  greater  advantage. 
The  motor  cylinder  is  surrounded  by  water  for  cooling,  but  this 
is  unnecessary  with  the  displacer,  as  it  uses  only  cool  gases.  The 
pressures  used  are  high,  so  that  both  motor  piston  and  its  connec- 
tions are  made  very  strong  ;  the  pressure  on  the  displacer  piston 
is  very  little,  so  the  connections  are  light.  It  is  not  a  compressing 
pump,  and  is  not  intended  to  compress  before  introduction  into 


Gas  Engines  of  Different  Types  in  Practice         1 87 

the  motor,  but  merely  to  exercise  force  enough  to  pass  the  gases 
through  the  lift  valve  into  the  motor  cylinder,  and  there  displace 
the  burnt  gases,  discharging  them  into  the  exhaust  pipe.  The 
pressure  to  be  overcome  is  only  that  due  to  resistance  in  the 
exhaust  pipes  and  the  lift  valve. 

The  inlet  valve  for  gas  and  air  is  also  automatic  ;  its  seat  is  of 
the  usual  conical  kind  but  somewhat  broad.  A  gutter  runs  round 
the  centre,  having  small  holes  bored  through  to  a  recess  behind, 


FIG.  57. — Longitudinal  Section  of  Clerk  Gas  Engine. 

which  communicates  with  the  gas  supply  pipe.  The  suction  lifts  the 
valve  to  a  certain  height,  and,  as  the  gases  enter,  the  air  flows  past 
the  holes  and  becomes  thoroughly  impregnated  with  gas,  the 
extent  being  determined  by  the  number  of  holes  and  the  propor- 
tion of  their  area  to  the  total  area  of  the  valve  opening.  The 
upper  valve  is  made  heavy  to  withstand  the  maximum  pressure  of 
the  explosion  ;  both  valves  are  arranged  so  that  the  guide  forms 


1 88  The  Gas  Engine 

a  piston  working  in  an  air  cylinder,  so  arranged  as  to  check 
the  fall  of  the  valve  before  touching  the  seat,  and  so  prevent  any 
disagreeable  rattle. 

Description  of  the  Drawings. — Fig.  56  is  a  general  view  of  the 
engine.  Fig.  573  longitudinal  section.  Fig.  58  a  sectional  plan. 
In  these  drawings  all  the  essential  parts  of  the  engine  are  repre- 
sented ;  the  sectional  plan  (fig.  58)  shows  the  two  cylinders, 
motor  A  and  displacer  B,  in  which  work  the  pistons  c  and  D 
suitably  connected  to  cranks  not  shown  in  the  drawing,  but  on  a 
common  crank  shaft.  The  motor  crank  is  double  and  of  great 
strength  ;  the  displacer  crank  pin  is  fixed  into  an  arm  in  the  fly- 
wheel, and  in  the  direction  of  motion  of  the  engine  is  a  half  right 
angle  or  quarter  circle  in  advance.  The  motor  piston  is  shown  at 
its  extreme  out-stroke,  having  passed  over  the  exhaust  ports  E  E1, 
the  piston  thus  serving  as  its  own  exhaust  valve,  and  dispensing 
with  any  other,  as  shown  ;  the  displacer  piston  has  moved  half  in 
and  discharged  a  portion  of  the  contents  through  the  valve  F 
(more  distinctly  seen  in  the  other  section,  fig.  57)  into  the  conical 
space  G,  which  is  so  proportioned  that  the  entering  gases  push 
before  them  the  burned  gases  through  the  ports  referred  to,  but 
without  following  them  into  these  ports.  By  the  continued  move- 
ment, all  the  gases  in  B  pass  into  A  and  the  space  G;  the  capacities 
of  the  two  cylinders  are  so  related  that  as  much  as  possible  of  the 
burnt  gases  is  discharged  into  the  atmosphere,  but  without  carrying 
away  any  of  the  fresh  mixture  containing  unburned  gas;  this  neces- 
sitates the  mixture  next  the  piston  being  somewhat  more  dilute 
than  that  next  the  inlet  valve,  but  the  commotion  occasioned  by 
compression  so  far  equalises  this  undesirable  state  of  things  that  at 
half  in-stroke  the  mixture  in  its  weakened  portions  is  quite  capable 
of  inflammation  by  a  light  or  the  electric  spark.  The  piston  D 
having  completed  its  in-stroke,  c  has  passed  over  the  discharge 
ports  and  compresses  the  contents  of  the  cylinder  into  the  space  G; 
when  full  in  and  therefore  completely  compressed,  the  slide  valve 
M  has  moved  into  such  a  position  as  to  ignite  the  mixture  ;  the 
maximum  pressure  being  attained  very  rapidly  and  before  the 
piston  can  move  appreciably  on  its  out-stroke,  the  piston  is  impelled 
forward  under  the  pressure  produced  until  it  reaches  the  ports  E  E1, 
when  the  contents  are  rapidly  discharged,  and  the  interior  and 


Gas  Engines  of  Different  Types  in  Practice 


190 


The  Gas  Engine 


exterior  pressures  equalised.  Meantime  the  piston  D  being  in 
advance  of  the  motor  has  moved  to  the  end  of  its  stroke  and  is 
beginning  to  return,  it  has  charged  the  cylinder  B  with  a  mixture 
of  gas  and  air  from  the  automatic  valve  H  (fig.  57),  the  commu- 
nication being  made  by  the  pipe  w  (fig.  58).  In  the  seat  of  this 
valve  are  bored  a  number  of  small  holes  passing  into  the  annular 
ypace  K  K1  (fig.  57),  which  communicates  with  the  gas  cock  L  (fig. 
58)  through  the  passage  shown,  in  which  is  situated  the  lift-govern- 
ing valve,  not  seen.  Under  the  deficit  of  pressure  caused  by  the 


~'//'/'W/\  Upper  lift  valve. 


•| Quieting  piston. 


'/A Lower  lift  valve. 


-•;f3_.  Gas  channel. 


Quieting  piston. 


FIG.  59.— Section  of  Lift  Valves.     Clerk  Engine. 


movement  of  the  displacing  or  charging  piston,  the  valve  is  lifted 
and  the  exterior  atmosphere  rushes  through,  at  the  same  time  the 
gas  passing  through  the  holes  mixes  with  it  thoroughly,  the  pro- 
portion being  determined  by  the  relative  areas  of  the  holes  and 
the  space  available  for  air  by  the  lift  of  the  valve. 

The  gases  in  B  are  under  some  slight  compression  before  the 
complete  discharge  of  A,  but  not  sufficiently  great  to  cause  any 
material  resistance  ;  so  soon  as  the  pressure  under  the  valve  F  is 
slightly  in  excess  of  that  above  it,  then  it  lifts  and  the  gases  pass 
into  G.  The  passage  from  the  valve,  which  may  be  called  the 


Gas  Engines  of  Different  Types  in  Practice         191 


upper  lift  valve,  is  more  clearly  seen  in  fig.  57  :  the  igniting  hole 
is  shown  at  N,  and  communicates  at  the  proper  time  with  flame  in 
the  cavity  o,  which  has  been  ignited  at  the  exterior  flame  P,  from 
a  Bunsen  burner  (fig.  58). 

The  two  automatic  valves  charging  the  displacer  cylinder  and 
discharging  into  the  motor  cylinder  are  provided  with  quieting 
pistons,  cushioning  the  blow  on  the  valve  seat  and  preventing 
rattle  ;  they  are  similar  to  the  dash  pot  contrivances  used  on 
Corliss'  steam  engines  to  check  the  snap  of  the  steam  valves,  but, 
unlike  them,  are  attached  directly  to  the  valve,  instead  of  to  the 
valve  spindle  and  guide.  The  arrangement  is  very  clearly  seen  at 
fig.  59  :  the  lower  valve  has  no  spring,  it  returns  to  its  seat  by  its 
own  weight ;  but  the  upper  valve  requires  to  act  more  quickly 
and  is  pulled  down  by  a  spring. 

The  piston  attached  compresses  the  air  before  it,  and  the 
valve  strikes  its  seat  rapidly  but  without  jar  or  recoil. 

The  igniting  slide,  M,  is  driven  from  an  eccentric  on  the  crank 
shaft  through  a  bell  crank  and  guide. 

Diagrams  and  Gas  Consumption. — The  followingxtests  give 
the  latest  results  from  the  Clerk  engine  ;  they  are  the  usual  trials 

TESTS  OF  THE  CLERK  ENGINES  OF  VARIOUS  POWERS. 


2   HP 

4  HP 

6  HP 

8  HP 

12  HP 

Diameter  of  motor  cylinder        .   :    5  ins. 

6  ins.        7  ins. 

8  ins. 

9  ins. 

Stroke         .....       8  in>. 

10  ins.      12  ins. 

16  ins. 

20  ins. 

Diameter  of  displacer  cylinder  . 

6  ins. 

7  ins.        7.5  ins. 

i  D  ins. 

10  ins. 

Stroke         .         .         .         .         •   i    9  ins. 

ii  ins.      12  ins. 

13  ins.      20  ins. 

Average  revs,  per  min.  during  test         212 

190           146 

142 

132 

Average  pressure   (available)   in 

motor  cylinder  in  Ibs.  persq.  in. 

43  '2 

63  '9 

53*a 

60-3 

64-8 

Power  indicated  in  motor  cylinder 

3-62 

8  68          9-05 

17-38 

27-46 

Power  by  dynamometer 

270 

5  '63          7  '23 

7 

13-69 

23  21 

Gas  consumption  in  cb.   ft.   per 

IHP  per  hour          .         .         .       29-8 

24-I9        24-3 

20-94 

20-39 

Gas  consumption  per  brake  HP 

hour        .....        40-0 

37  '3          30  "42 

26-58 

24-12 

Max.  pressure  of  explosion  in  Ibs. 

per  sq.  in.  above  atmos.          .   !  155  Ibs. 

236 

i95 

*95 

238 

Pressure  of  compression  in  Ibs. 

per  sq.  in.  above  atmos.          .  j    38  Ibs. 

55             48 

49 

57 

Displacer  resistance  . 

O'4o 

o  So    i       o  86 

I'5° 

2"OO 

Gas  consumed  per  hour  bv  each  ; 

engine  at  spec  i  without  load  .     40  cb  ft. 

58  cb  ft. 

57  cb.  ft. 

70  cb.  ft.  90  cb.  ft. 

192 


The  Gas  Engine 


made  by  Messrs.  L.  Sterne  and  Co.  on  all  engines  before  leaving 
the  works,  and  therefore  represent  fairly  the  economy  to  be 
expected  from  these  engines  in  ordinary  work.  They  are  from 
2.  4,  6.  8,  and  12  HP  engines  (nominal).  The  trials  were  made 
during  1885  at  the  Crown  Iron  Works,  Glasgow,  under  the 
direction  of  Mr.  G.  H.  Garrett. 

Figs.  60,  61,  62  are  fair  samples  of  the  diagrams  taken  during 
the  tests.  Figs.  63  and  64  are  diagrams  from  the  displacers 
showing  the  displacer  resistance. 


Nominal  HP,  6  ;  diam.  of  cylinder.  7"  ;  length  or  stroke,  12"  ;  No.  of  revs.  146  ;  in- 
dicated HP,  9  05  :  consumpt.  per  1HP,  24  '30  cb.  ft.  ;  consumpt.  loose,  57  cb.  ft.  ; 
brake  HP,  7^23  ;  consumpt.  per  BHP,  30^42  cb.  ft.  ;  mean  pressure,  53'2  Ibs.  :  max. 
pressure,  195  Ibs.  ;  press,  before  ignition,  48  Ibs.  ;  scale  of  spring,  jjg"  per  Ib. 

FIG.  60.—  Diagram  from  Clerk  Gas  Engine,  6  HP. 

Calculating  from  these  diagrams  the  actual  indicated  efficiency 
it  comes  to  16  per  cent  of  the  total  heat  given  to  the  engine. 

The  compression  space  in  the  Clerk  engines  is  as  nearly  as 
possible  one-half  of  the  volume  swept  by  the  piston  from  the 
exhaust  port  to  the  end  of  its  stroke.  The  theoretic  efficiency  is 
therefore 


The  compression  is  higher,  and  therefore  the  tneoretic  efficiency 


Gas  Engines  of  Different  Types  in  Practice         193 

of  this  engine  is  higher  than  the  Otto,  but  the  difficulties  of  pro- 
portioning the  two  cylinders  of  the  Clerk  engine  cause  a  small 
loss  of  unburned  gas  at  the  exhaust  ports,  so  that  the  actual 
efficiency  is  similar  to  that  of  Otto. 

The  mixture  sent  from  the  displacer  cylinder  into  the  motor 
and  the  space  at  the  end  of  it,  contains  8  vols.  of  air  with 
i  vol.  of  coal  gas,  but  on  passing  through  the  upper  lift  valve 
and  mixing  to  some  extent  with  the  exhaust  there  contained,  it 
is  somewhat  diluted;  the  heat  acquired  by  contact  with  the 
products  of  combustion  and  with  the  sides  of  the  cylinder  expands 
the  entering  gases,  and  a  temperature  of  not  less  than  100°  C.  is 


Nominal  HP,  8  :  diam.  of  cylinder,  8''  :  length  of  stroke,  16"  ;  No.  of  revs.  142  : 
indicated  HP,  i7'38;  consumpt.  per  IHP,  20*94  cb.  ft.  :  consumpt.  running  light 
per  hour,  70  cb.  ft.  ;  brake  HP,  I3'6g  ;  consumpt.  per  BHP,  26*58  cb.  ft.  ;  mean 
pressure,  60*3  Ibs.  ;  max.  pressure,  195  Ibs. ;  pressure  before  ignition,  49  Ibs.  ;  scale 
of  spring,  TJ-5"  per  Ib. 

FIG.  61. — Diagram  from  Clerk  Gas  Engine,  8  HP. 

attained  before  the  compression  commences.  The  result  of  this 
is,  that  the  displacer  gases,  being  expanded,  expel  more  of  the 
exhaust  gases  through  the  discharge  ports  than  would  appear 
from  the  volume  swept  by  the  displacer  piston.  This  volume  is 
equal  to  the  volume  swept  by  the  motor  piston,  from  closing  of 
the  exhaust  ports  to  complete  in-stroke.  If  no  expansion  and  no 
mixing  occurred,  the  exhaust  gases  contained  in  the  compression 
space  would  remain  in  front  of  the  cooler  explosive  charge  •  but 
the  heat  increases  the  volume  at  least  one-third,  so  that  the 

o 


194 


The  Gas  Engine 


volume  occupied  will  be  i^  times  the  volume  swept  by  either 
piston.     The  volume  of  cylinder  plus  space  is  i^  vol.  of  cylinder, 


I 
J 

• 


so  that  the  actual  exhaust  gases  present  are  £  vol.,  or  T^  of  the 
total  gases  present.  But  mixing  must  occur  to  a  considerable 
extent  and  be  made  very  complete  on  the  return  stroke  during 


Gas  Engines  of  Different  Types  in  Practice         195 


compression.  The  result  of  all  this  is  the  production  of  an  explosive 
mixture  which  is  explosive  in  every  part  of  it,  and  of  an  average 
composition  of  one  volume  of  coal  gas  in  ten  of  the  mixture. 
The  proportion  of  burned  gases  present  is  very  slight ;  the  only 
reason  why  any  should  be  left  is  the  necessity  of  preventing  any 


\ff 


E        ~ 


Li 


W 


a        '-n 

&    Q 


appreciable  discharge  of  unburned  gas  at  the  exhaust  ports.     The 
mixture  used  is  a  comparatively  rich  one. 

Tangye  Engine. — Messrs.  Tangye,  of  Birmingham,  have  pro- 
duced an  engine  in  which  compression  of  the  kind  common  to  the 
third  type  is  used  and  an  ignition  is  obtained  for  every  revolution 


1 96  The  Gas  Engine 

when  at  full  power.  It  is  Robson's  patent  and  contains  only  one 
cylinder.  All  the  necessary  operations  of  charging,  compressing, 
and  igniting  are  fulfilled  with  one  cylinder  ;  it  is  arranged  as  in  an 
ordinary  steam  engine.  The  front  end  of  the  cylinder  unlike  the 
Otto  and  Clerk  engines  is  closed,  the  piston  being  provided  with 
a  piston  rod,  cylinder  cover,  and  stuffing  box,  as  in  steam.  The 
front  end  of  the  cylinder  serves  for  charging,  the  back  end  for 
compression  and  explosion. 

There  is  a  compression  space  at  the  back  end  of  the  cylinder  as 
in  the  other  engines. 

The  action  is  as  follows.  During  the  return  stroke,  gas  and 
air  mixture  is  drawn  into  the  front  end  of  the  cylinder  at  atmo- 


FIG.  65.— Robson's  Gas  Engine. 

spheric  pressure,  through  an  automa'  ic  valve.  The  next  out-stroke 
compresses  the  mixture  into  a  large  intermediate  chamber  at  a 
pressure  of  not  more  than  five  Ibs.  per  sq.  in.  above  atmosphere. 
When  full  out  and  the  exhaust  ports  therefore  open,  this  pressure 
lifts  a  valve  leading  into  the  compression  space  of  the  engine,  dis- 
charging before  it  the  gases  contained  in  the  cylinder  through  the 
exhaust  valve  and  filling  the  cylinder  and  space  with  explosive 
mixture.  This  reduces  the  pressure  in  the  intermediate  reservoir 
to  atmosphere  so  that  the  next  in-movement  of  the  piston  com- 
presses the  explosive  mixture  upon  one  side  of  the  piston  and  takes 
in  fresh  mixture  on  the  other  side. 


Gas  Engines  of  Different  Types  in  Practice         197 

When  compression  is  completed  the  igniting  valve  acts  and  the 
explosion  impels  the  piston ;  so  soon  as  the  exhaust  ports  open,  the 
pressure  falls  to  atmosphere,  and  then  the  reservoir  pressure  being 
superior  to  that  in  the  cylinder,  the  automatic  valve  acts  and  the 
fresh  charge  enters. 

Thus  an  explosion  is  obtained  at  every  revolution  by  using  the 
front  end  of  the  cylinder  as  displacer  and  storing  up  the  pressure 
in  an  intermediate  reservoir. 

The  governing  is  managed  by  cutting  oft"  gas  supply,  but  is 
hampered  considerably  by  the  intermediate  chamber.  Fig.  65 
is  an  external  view  of  the  engine,  which  is  exceedingly  neat  and 
of  substantial  workmanship. 

Tfie  Stockport  Engine. — This  engine  is  similar  to   Robson's 


FIG.  66. — The  Stockport  Gas  Engine. 

in  theory  but  the  front  end  of  the  cylinder  is  not  used  for  charg- 
ing, the  piston  being  made  a  double  trunk  with  the  crank  be- 
tween, and  one  end  and  one  cylinder  being  motor,  the  other  end 
and  the  other  cylinder  being  displacer.  Compression  occurs  in 
the  motor  cylinder.  Fig.  66  shows  the  external  appearance.  The 
valve  arrangements  differ  from  those  of  Messrs.  Tangye.  It  is 
made  by  Messrs.  Andrew,  of  Stockport. 

Atkinson's  Differential  Engine. — The  description  of  engines  of 
this  type  would  be  incomplete  without  mention  of  this  engine, 
exhibited  at  the  Inventions  Exhibition  for  the  first  time  in  1885. 
It  is  exceedingly  ingenious  and  quite  novel. 


198  The  Gas  Engine 

Fig.  67  is  an  elevation,  fig.  68  a  section,  and  fig.  69  a  plan. 
The  action  is  very  clearly  seen  from  the  different  positions  on  fig. 
70. 

The  same  cylinder  serves  for  all  purposes  of  the  cycle  ;  two 
trunk  pistons,  working  in  opposite  ends  of  it  are  connected  to 


ELEVATION 


FIG.  67. — Atkinson's  Differential  Gas  Engine. 

the  levers  and  from  thence  to  the  crank  shaft  by  the  connecting 
rods.     The  short  rods  cause  the  necessary  actions. 

In  the  first  position,  fig.  70,  the  pistons  are  at  one  extreme  of 
their  stroke,  and  are  just  beginning  to  separate.     The  charge  of  gas 


Gas  Engines  of  Different  Types  in  Practice         159 

and  air  enters  between  them  through  the  automatic  lift  valve,  and 
in  position  2,  the  charge  has  entered  and  the  further  movement 
of  the  piston  is  about  to  close  the  port  leading  to  the  admission 
and  exhaust  valves.  The  compression  thus  commences  and  in 
position  3  it  is  completed.  The  ignition  occurs  and  the  pistons 


SECTION 


FIG.  68.— Atkinson's  Differential  Gas  Engine. 

now  rapidly  separate,  the  exhaust  port  being  uncovered  and  the 
discharge  commencing  in  position  4.  By  this  clever  method 
the  whole  operations  of  admission,  discharge,  ignition,  and  expan- 
sion are  performed  in  the  single  cylinder  with  only  two  automatic 


200 


The  Gas  Engine 


FlG.  69.— Atkinson's  Differential  Gas  Engine. 

X  "^  * 

\  1. 


FlG.  70. — Atkinson's  Differential  Gas  Engine. 


Gas  Engines  cf  Different  Types  in  Practice         201 

valves  which  are  never  exposed  to  the  pressure  of  explosion,  the 
pistons  acting  in  some  part  as  valves  and  uncovering  the  exhaust 
and  inlet  ports  when  required.  In  the  other  extreme  position  they 
also  act  as  valves,  the  outside  piston  uncovering  the  igniting  port 
at  the  correct  time.  Sufficient  experience  has  not  yet  been  accu- 
mulated with  this  engine  to  speak  positively  as  to  its  performance. 
To  the  author,  the  principal  disadvantage  appears  to  lie  in  a  com- 
pression space  of  diameter  so  great,  in  proportion  to  depth,  that  the 
ratio  of  cooling  surface  to  volume  of  hot  gases  is  largely  in  excess 
of  that  common  to  other  engines.  This  disadvantage  will  diminish 
the  economy  which  the  great  expansion  would  otherwise  give. 


202  TJie  Gas  Engine 


CHAPTER  VIII. 

IGNITING   ARRANGEMENTS. 

HOWEVER  perfect  the  theoretic  cycle  of  an  engine,  or  however 
admirable  is  its  construction,  in  the  absence  of  a  good  igniting 
valve  the  skill  and  energy  expended  is  of  no  avail.  The  engine 
is  a  useless  mass  of  metal  requiring  power  to  move  itself  rather 
than  furnishing  power  to  set  other  machines  in  motion. 

In  the  earlier  stages  of  gas  engine  manufacture,  the  igniting 
method  has  been  the  most  fruitful  source  of  annoyance  and  diffi- 
culty ;  even  yet,  after  many  years  of  engineering  experience,  the 
igniting  valve  is  still  the  initial  difficulty  which  the  inventor  must 
overcome  before  he  gets  the  opportunity  of  testing  his  theories  of 
heat  and  work  in  a  moving  machine.  Quite  a  number  of  wit- 
nesses, in  the  shape  of  unworkable  gas  engines,  in  many  engineers' 
workshops  throughout  Britain,  attest  silently  but  emphatically  the 
difficulties  of  the  igniting  valve. 

The  problem  is  by  no  means  a  simple  one,  and  the  care 
lavished  upon  its  solution  would  not  be  suspected  on  inspection 
of  the  igniting  gear  of  any  good  modern  engine.  Much  has  been 
done,  but  much  still  remains  yet  to  be  accomplished  before  flame 
is  as  completely  and  effectively  under  control  as  steam. 

In  the  noncompression  engines  the  problem  is  comparatively 
simple — to  inflame  a  volume  of  explosive  mixture  enclosed  in 
a  cylinder,  so  that  the  explosion  is  confined  within  the  cylinder, 
and  no  communication  is  open  to  atmosphere.  This  is  to  be  re- 
peated regularly  and  with  certainty  at  rates  varying  from  60  to  150 
times  per  minute,  depending  upon  the  speed  of  the  engine.  In 
the  earlier  trials,  what  may  be  called  the  touch  hole  method 
naturally  suggested  itself ;  the  piston  after  taking  in  its  charge, 
crossed  a  small  hole  and  sucked  a  flame  through  it  into  the  cylin- 


Igniting  Arrangements  203 

der,  the  hole  being  either  small  enough  to  occasion  no  substantial 
loss  of  pressure  upon  explosion,  or  covered  by  a  small  valve  closing 
with  the  pressure  from  the  interior.  This  is  the  earliest  flame 
method.  Then  comes  the  idea  of  using  the  electric  spark,  and  so 
completely  closing  up  the  cylinder,  and,  later  on,  a  return  to  flame, 
using  a  double  flame,  one  to  ignite  an  intermediate  one,  and 
the  intermediate  flame  carried  in  a  pocket  or  hollow  cock  to  the 
mixture.  Then  the  idea  of  spongy  platinum  suggested  by  the 
well-known  Dobermer's  Hydrogen  lamp.  Later  on  the  heating  of 
metal  tubes  or  metal  masses  and  the  ignition  of  the  gases  by  con- 
tact with  them.  Then  electrical  ignition  again,  but  this  time  by 
heating  a  platinum  wire  to  incandescence.  All  those  methods  were 
proposed  and  to  some  extent  practised  long  before  gas  engines 
appeared  in  any  commercially  successful  form. 

Ignition  methods  may  be  classed  in  four  distinct  groups. 

(1)  Electrical  methods. 

(2)  Flame  methods. 

(3)  Incandescence  methods. 

(4)  Methods  depending  on  '  Catalytic  '  or  chemical  action. 

(i)  ELECTRICAL  METHODS. 

Spark  Method. — The  use  of  the  electric  form  of  energy  seems 
at  first  sight  a  very  convenient  and  easy  method  of  getting  an  in- 
tense heat  at  any  desired  time  and  in  any  desired  spot  in  the 
interior  of  a  cylinder.  The  electric  spark  has  long  been  used  by 
chemists  to  explode  the  contents  of  the  eudiometer  in  which  gas 
analysis  is  effected  ;  and  the  platinum  wire  rendered  incandescent 
by  the  current  from  a  battery  has  long  been  familiar  to  experi- 
menters and  is  used  by  them  for  many  purposes.  The  spark 
method  was  used  in  the  Lenoir  engine.  A  Bunsen's  battery,  a 
Rumkorff  induction  coil,  and  a  commutator  or  distributor,  is 
required  in  addition  to  the  insulated  points  between  which  the 
spark  passes  in  the  interior  of  the  cylinder. 

Fig.  71  is  drawn  to  show  clearly  the  general  arrangement. 
The  Bunsen's  battery  A  generates  the  current,  which  passes  by  the 
wires  to  the  coil  B,  from  which  the  intensity  current  passes  to  the 
insulated  points  D  D  by  way  of  the  distributor  c.  The  negative 
pole  of  the  coil  is  permanently  connected  to  any  part  of  the  metal 


204 


The  Gas  Engine 


work  of  the  engine  ;  the  igniting  points  D,  D,  consist  of  porcelain 
plugs  seen  on  a  larger  scale  at  E.  The  porcelain  is  firmly 
cemented  into  the  brass  nut  i,  and  the  wire  2  which  passes  through 
a  hole  in  the  plug  terminates  outside  in  the  connecting  screw  3, 
and  inside  is  bent  over  the  end  of  the  plug ;  the  other  wire  4, 
passes  through  another  hole  in  the  plug,  is  bent  over  in  the  inside 
lying  near  the  wire  2  but  not  touching  it,  it  then  passes  through 
the  side  of  the  plug  touching  the  metal  of  the  nut.  When  the 
nut  is  screwed  into  position  the  one  wire  is  in  metallic  connec- 
tion with  the  cylinder  of  the  engine,  and  the  other  is  insulated 
from  it. 


FIG.  71. — Ignition  Arrangements  Lenoir  Engine. 

The  distributor  c  consists  of  an  insulated  metallic  arm  i  rota- 
ting on  the  end  of  the  crank  shaft  over  the  insulated  ring  2,  whicl: 
is  connected  to  the  positive  pole  of  the  coil.  Two  insulated  seg- 
ments 3,  4,  are  connected  by  wires  to  the  igniting  plugs  D,  D  ;  in 
rotating,  the  arm  i  comes  alternately  over  3,  4,  and  it  is  within 
sparking  distance  of  the  ring  2  as  well  as  the  segments ;  the  sparks 
pass  alternately  to  the  segments  and  thence  alternately  to  the 
opposite  ends  of  the  cylinder.  The  ebonite  disc  carrying  the  seg- 
ments and  ring  is  so  adjusted  that  the  spark  begins  to  pass  at  either 
end,  just  as  the  admission  valve  closes.  If  it  passed  too  soon  the 


Igniting  Arrangements  205 

explosion  would  occur  before  the  admission  valve  closed,  and 
therefore  would  partly  be  lost,  and  at  the  same  time  would  make  a 
disagreeable  noise.  If  it  is  passed  too  late,  power  is  lost,  because 
the  piston  is  at  its  most  rapid  rate  of  movement  and  is  reducing 
the  pressure  of  the  cylinder  contents  uselessly. 

Notwithstanding  the  most  careful  adjustment,  some  time 
elapses  between  the  closing  of  the  admission  valve  and  the  explo- 
sion. When  all  is  in  good  order  this  arrangement  works  very  well, 
but  should  the  insulation  be  disturbed  and  any  short  circuiting 
occur,  the  spark  fails  to  pass  between  the  points  in  the  interior  of 
the  cylinder  and  a  missed  or  late  ignition  results.  This  often 
happens  in  starting  the  engine  when  it  is  cold ;  the  first  few  explo- 
sions cause  a  condensation  of  water  upon  the  points  and  the  spark 
then  fails,  the  current  passing  through  the  water  film  from  wire  to 
wire  without  spark.  The  igniters  then  require  to  be  uncoupled 
and  dried.  To  reduce  this  trouble,  the  points  are  kept  towards 
the  top  of  the  cylinder  in  the  end  covers  so  that  any  water  or  oil 
drainage  may  flow  down  and  leave  them  dry.  The  difficulties  of 
insulation,  coil  and  battery,  are  so  great  that  they  did  much  to 
prevent  the  use  of  the  Lenoir  engine  ;  unless  the  machine  fell  into 
intelligent  hands  it  was  sure  to  go  wrong  and  give  trouble. 

The  spark  method  has  never  been  applied  to  compression 
engines  as  the  compression  increases  all  difficulties.  The  Lenoir 
igniting  plug,  or  '  inflamer '  as  it  was  called,  if  put  in  a  compression 
engine  leaks  badly  and  cannot  be  got  to  act  efficiently.  Many 
specifications  of  compression  and  other  engines  state  that  ignition 
is  accomplished  by  the  electrical  spark,  but  the  Lenoir  engine  alone 
attained  any  success. 

Incandescent  Wire  Method, — This  method  very  naturally  suggests 
itself  as  a  solution  of  the  difficulties  of  the  high  tension  spark;  the 
coil  is  dispensed  with  and  the  current  from  the  battery  is  applied 
directly  to  heat  a  thin  platinum  wire.  The  difficulty  of  insulating  is 
very  slight.  The  tension  being  low  it  is  a  matter  of  indifference 
whether  the  insulating  material  is  wetted  or  not.  The  wire  being 
constantly  at  a  red  heat  cannot  remain  at  all  times  in  the  cylinder, 
but  is  put  into  communication  with  it  at  proper  times  by  means  of 
a  slide  valve.  Fig.  72  is  a  drawing  of  an  igniting  slide  of  this  kind, 
as  used  by  the  author  in  experimental  work.  It  acts  very  well  in- 


zo6 


The  Gas  Engine 


deed.  The  screw  i  carries  the  insulated  rod  2,  insulated  by  means 
of  asbestos  card-board  packed  into  the  space  and  screwed  down 
firmly  by  the  screw  3.  The  other  wire  is  screwed  into  the  metal 
and  so  is  in  metallic  connection  with  the  metal  work  of  the 
engine.  One  wire  from  the  battery  connects  to  any  portion  of  the 
engine  ;  the  other  is  insulated.  The  platinum  wire  4  is  thus  kept 
continually  at  a  red  heat,  and  the  slide  5  moving  at  proper  times 
causes  the  gases  to  be  ignited  to  flow  into  the  chamber  containing 
the  platinum  spiral,  by  the  hole  6,  and  so  causes  the  explosion. 


FlG.  72.  — Electrical  Igniting  Valve  (Clerk). 
Incandescent  Platinum  Wire. 


There  is  only  one  precaution  required  in  using  this.  The  battery 
must  not  be  too  powerful  ;  if  the  wire  is  heated  by  it  to  near  its 
fusing  point,  then  the  further  heat  supplied  by  successive  explo- 
sions may  cause  its  destruction.  It  requires  to  be  kept  at  a  good 
red  heat  and  no  more  when  open  to  the  air  :  when  closed  up  and 
in  contact  with  the  hot  gases  it  will  then  become  almost  white  hot  ; 
anything  above  this  may  fuse  it.  The  battery  is  of  course  at  all 
times  a  source  of  care  ;  as  it  requires  to  be  often  renewed,  it  is  only 
for  experimental  work  that  this  arrangement  answers  well.  In  the 
hands  of  the  general  public  it  would  come  to  grief.  Hugon  and 


Igniting  Arrangements  207 

many  others  proposed  similar  arrangements,  but  they  do  not  appear 
to  have  worked  them  out. 

Arc  Method. — There  is  another  electrical  method.  A  small 
dynamo  attached  to  the  engine  keeps  up  a  continuous  current  and 
heavy  platinum  points  in  communication  with  the  cylinder  carry 
the  arc.  This  is  difficult,  however,  as  the  points  constantly  volatilize 
and  require  frequent  renewal.  This  method  has  never  come  into 
practical  use  ;  it  is  described  several  .times  in  specifications. 


(2)  FLAME  METHODS. 

The  earliest  really  efficient  igniting  valve  is  that  described  by 
Barnett  in  his  specification  of  1838.  It  is  the  parent  form  of 
the  most  extensively  used  valve,  the  'Otto.' 

Barnetfs  Igniting  Valve. — Fig.  73  shows  a  vertical  section  and  a 
plan  of  this  valve.  It  consists  of  a  conical  stopcock  with  a  hollow 
plug;  the  shell  contains  two  ports,  i  and  2 — i  open  to  the  atmosphere 
and  2  communicating  with  the  cylinder.  The  plug  of  the  cock  has 
one  port,  3,  so  arranged  that  it  may  open  on  the  atmosphere  port  or 
the  cylinder  port  in  the  shell,  but  cover  enough  being  left  to  pre- 
vent it  opening  to  both  at  the  same  time.  In  turning  round  it 
closes  on  the  atmosphere  before  opening  to  the  cylinder. 

A  gas  jet  burns  at  the  bottom  of  the  shell,  and  in  the  hollow 
of  the  plug,  the  ports  3  and  i  being  long  enough  and  wide  enough 
to  allow  the  air  free  circulation  as  shown  by  the  arrows.  The  flame 
must  not  be  too  large  or  it  will  fill  the  whole  interior  with  gas  and 
prevent  air  getting  in;  the  flame  will  then  burn  at  the  port  i  in  the 
air  and  will  not  enter  the  cock.  Suppose  it  to  be  burning  regu- 
larly in  the  cock  as  shown  in  the  drawing,  then  if  the  plug  is  suddenly 
turned  round  so  that  port  3  closes  upon  the  atmosphere  port  i, 
and  opens  upon  the  cylinder  port  2,  the  air  supply  will  be  sufficient 
to  keep  the  flame  living  till  the  mixture  contained  in  2  reaches  it 
The  explosion  then  occurs.  The  port  2  is  of  the  same  shape  as  i, 
so  that  the  flame  causes  the  gases  to  circulate  the  same  as  the  air 
did  when  open  to  it  ;  the  mixture  comes  in  contact  with  the  flame 
by  circulating  through  the  plug.  If  the  port  2  is  made  so  small 
that  no  circulation  occurs,  then  the  ignition  will  be  a  very  uncer- 
tain matter  ;  as  the  gases  will  require  to  get  at  the  flame  by  diffu- 


208 


The  Gas  Engine 


sion,  which  is  a  slow  process,  and  the  flame  may  be  extinguished 
before  they  arrive  at  it.  The  explosion  of  course  extinguishes  the 
flame,  but  when  the  plug  is  again  rotated  to  open  to  the  air,  the 
external  flame  relights  it  and  it  is  ready  for  another  ignition. 


FIG.  73.— Barnett's  Igniting  Valve  (flame). 


Igniting  Arrangements  209 

Hugon's  Igniting  Valve. — In  the  small  Hugon  engine  Barnett's 
method  was  first  applied  in  a  fairly  successful  manner. 

The  valve  is  shown  in  section  at  fig.  74. 

The  sectional  plan,  fig.  74,  shows  the  internal  flame  lit  and 
burning  in  the  ignition  port  i ;  the  external  flame  2  burns  close  to 
it  in  this  position,  so  as  to  be  ready  to  light  it  when  wanted.  The 
gas  for  the  internal  flame  is  supplied  under  higher  pressure  than 
that  of  the  ordinary  gas  mains  by  a  bellows  pump  and  small 
reservoir  through  the  flexible  rubber  pipe.  For  the  external  flame 
the  gas  is  used  at  the  ordinary  pressure. 

When  ignition  is  required,  the  valve  moves  rapidly  forward 
causing  the  port  i  to  close  to  atmosphere  first,  and  then  to  open 
to  the  cylinder  port  3,  as  shown  at  the  other  end  of  the  slide. 

The  explosive  mixture  which  fills  the  port  3  is  at  once 
ignited  and  the  flame  finds  its  way  from  the  port  into  the  cylinder 
itself  ;  the  port  is  necessarily  filled  with  pure  explosive  mixture 
free  from  any  admixture  with  exhaust  gases,  as  all  the  mixture 
before  entering  the  cylinder  must  pass  through  it  and  so  sweep 
before  it  any  burned  gases  into  the  cylinder.  Hence  the  mixture 
in  the  port  will  be  more  ignitable  than  that  in  the  cylinder,  as  the 
mixture  there  is  diluted  in  part  with  exhaust  gases  while  that  in 
the  port  is  free  from  them. 

The  explosion  is  thus  exceedingly  certain  and  regular  ;  when 
it  occurs  it  extinguishes  the  internal  flame  and  at  the  same  time  its 
superior  pressure  forces  back  the  gas  in  the  rubber  pipe  while  the 
port  i  remains  open  to  the  cylinder. 

The  return  of  the  slide  again  opens  it  to  the  atmosphere,  and  here 
is  seen  the  necessity  of  using  the  gas  under  some  pressure.  Before  it 
can  relight  at  the  external  flame,  the  products  of  combustion  must  be 
expelled  from  the  gas  pipe  ;  if  the  gas  were  under  only  the  ordinary 
gas  main  pressure  there  would  be  no  time  for  this,  and  the  valve 
would  return  to  ignite  without  a  flame.  The  expedient  of  increas- 
ing pressure  is  somewhat  clumsy  but  it  acts  fairly  well.  The  port  i 
is  made  large  to  give  space  for  the  air  necessary  to  support  the 
flame  while  the  ignition  port  is  passing  from  atmosphere  to 
cylinder  port.  At  the  moment  of  explosion,  the  cylinder  is  com- 
pletely closed  from  the  air. 

p 


210 


The  Gas  Engine 


The  explosion  is  therefore  completely  contained  within  the 
cylinder  and  no  sound  is  heard. 


& 


I 

K 


d 

£ 


Igniting  A  rrangements  2 1 1 

In  the  engine  at  the  Patent  Office  Museum  Mr.  S.  Ford  con- 
siderably improved  the  igniting  arrangement  by  intercepting  the 
rush  back  to  the  gas  pipe  by  a  light  check  valve  ;  he  was  thus  able 
to  use  gas  under  the  ordinary  gas  main's  pressure  and  dispense 
entirely  with  Hugon's  gas  pump  and  reservoir.  The  explosion,  in- 
stead of  forcing  a  considerable  volume  of  burned  gases  down  the 
gas  pipe,  simply  closed  the  check  valve,  which  opened  as  soon  as 
the  igniting  port  reached  the  air  again,  and  so  gave  the  gas  stream 
at  once. 

Otto's  Igniting  Valve. — The  igniting  valve  used  in  the  Otto 
and  Langen  engines  is  a  further  development  of  Barnett  and 
Hugon's  igniting  devices. 

As  applied  to  the  compression  engine  there  is  one  alteration, 
very  slight,  but  very  essential. 

In  the  Lenoir  and  Hugon  engines,  as  well  as  the  Otto  and 
Langen,  the  pressure  in  the  cylinder  is  the  same,  or  in  some  cases 
less  than  that  of  the  external  atmosphere,  that  is,  before  ignition. 
It  is  therefore  an  easier  matter  to  transfer  a  flame  burning  quietly 
in  the  air  to  the  cylinder  without  danger  of  extinction.  When  the 
gases  to  which  the  flame  is  to  be  transferred  exist  at  a  pressure 
some  40  to  50  Ibs.  per  square  inch  superior  to  that  of  the  flame 
itself,  it  is  not  so  easily  seen  how  the  flame  is  to  be  transferred 
without  extinction.  Generally  described  the  arrangement  is  as 
follows.  A  small  quantity  of  coal  gas  is  introduced  into  the 
upper  part  of  a  cavity  in  the  ignition  slide  ;  being  lighter  than  air  it 
remains  separate  from  it  and  has  no  tendency  to  mix  with  the  air 
beneath  it,  except  by  the  slow  process  of  gaseous  diffusion.  At  the 
surface  of  contact  with  the  air,  it  is  ignited  and  burns  with  a  blue 
flickering  flame.  The  movement  of  the  slide  cuts  off  communica- 
tion with  the  outer  atmosphere,  and  very  shortly  thereafter  opens 
on  the  admission  port  of  the  engine,  but  before  doing  this  it  opens 
on  a  small  hole  communicating  with  the  cylinder.  This  hole  com- 
municates with  the  gas  passage  in  the  upper  part  of  the  slide,  so 
that  the  gases  under  pressure  enter  and  force  the  gas  downwards, 
the  pressure  rising  in  the  port  more  slowly  than  would  occur  if 
the  main  port  opened  at  once.  The  pressure  is  therefore  nearly 
level  with  that  in  the  cylinder  when  the  main  port  opens,  and  the 
flame  still  burning  at  the  point  or  surface  of  junction  between  the 

P2 


212 


The  Gas  Engine 


gas  and  air,  ignites  the  mixture.  If  the  pressure  was  not  raised 
in  the  igniting  port  by  pressing  the  gas  downwards  and  thereby 
avoiding  a  rush  past  the  flame  portion,  the  rush  would  often 
extinguish  the  flame  and  an  ignition  would  be  missed.  The 
apparent  difficulty  of  transferring  the  flame  from  atmosphere  to 


FIG.  75. — Section,  Otto  Igniting  Valve. 

40  Ibs.  above  it  is  thus  simply  and  beautifully  overcome.  By  using 
a  portion  of  gas  in  the  upper  part  of  the  valve  cavity,  the  difficulty 
of  the  blow  back  of  explosion  down  the  gas  supply  pipe  is  also 
overcome,  as  the  gas  supply  can  be  cut  off  before  the  explosion  or 


Igniting  A  rrangements  2 1 3 

compression  pressure  comes  on.  It  is  cut  off  just  before  the  valve 
closes  the  flame  port  to  atmosphere. 

Fig.  75  is  a  vertical  section-showing  the  flame  cavity  in  the  slide, 
in  the  act  of  introducing  coal  gas  at  the  upper  part  and  inflaming 
it  at  the  point  of  junction,  between  gas  and  air. 

The  slide  A  contains  a  forked  passage  B  communicating  at  the 
lower  passage  with  the  air  inlet  c,  and  at  the  upper  passage  with 
the  funnel  F,  which  are  both  in  the  valve  cover  D,  which  holds  tl*e 
valve  against  the  engine  face.  The  jet  c  has  a  flame  constantly 
burning  into  the  funnel,  which  becomes  heated,  with  the  effect  of 
drawing  a  current  of  air  through  the  forked  passage  when  its  ports 
are  in  proper  position  ;  the  direction  of  the  current  is  shown  by 
the  arrows.  The  pipe  j  supplies  coal  gas  which  passes  along  the 
gutter  i,  cut  in  the  cover  and  valve  faces,  into  the  forked  passage 
c,  and  thence  to  the  funnel  F  where  it  is  inflamed  and  burns  as 
shown.  When  the  movement  of  the  slide  cuts  off  communication 
with  the  atmosphere,  it  also  closes  the  gutter  i  and  terminates  the 
supply  of  coal  gas  from  the  pipe  j,  but  the  upper  part  of  the  forked 
passage  contains  gas  ;  a  flame  therefore  flickers  as  shown.  Just 
before  B  opens  on  the  port  L,  fig.  76,  the  hole  K,  fig.  75,  opens  and 
the  pressure  from  the  explosion  space  causes  a  flow  into  B,  forcing 
before  it  the  gas  contained  in  the  hole,  thereby  intensifying  the 
flame  by  making  the  gas  pass  more  into  the  air  and  bringing  about 
the  equilibrium  of  the  pressures.  When  B  opens  on  L,  the  flame 
is  a  vigorous  one,  and  at  once  fires  the  whole  charge  in  the  explo- 
sion chamber.  Fig.  76  shows  the  slide  with  the  port  B  at  the 
moment  of  opening  on  L.  Fig.  77  is  an  end  elevation  of  the 
valve  and  cover,  showing  the  ports  and  gutters  dotted  and  lettered, 
position  same  as  in  fig.  76.  The  method  is  carried  out  completely 
and  is  a  very  perfect  one  indeed  ;  it  is  somewhat  slow  in  action, 
depending  as  it  does  on  a  proper  ventilation  of  the  forked  passage 
and  the  complete  replacement  of  the  burned  products  by  fresh  air 
before  the  gas  can  burn  properly  in  the  cavity.  If  the  engine  be 
run  more  rapidly  than  the  draught  of  the  funnel  can  clear  out  the 
passage  from  the  burned  gases,  then  the  flame  cannot  be  lit  in  it 
and  an  ignition  will  be  missed. 

It  is  a  method  exceedingly  successful  when  ignition  is  not 
required  too  frequently,  but  very  troublesome  and  uncertain  for 


214 


The  Gas  Engine. 


rapid  ignition.     The  Otto  and  Langen  engine  only  made  30  igni- 
tions per  minute,  and  the  Otto  compression  engine  makes  but  80 


FIG.  76.— Sectional  Plan,  Otto  Igniting  Valve. 


Igniting  A  rrangements 


215 


ignitions  per  minute  at  full  power  ;  its  efficiency  is  good  at  these 
rates,  but  at  150  per  minute  it  is  too  slow  in  action. 


I 

o 


to 

o 

6 


ClerKs  Igniting  Valve. — The  method  of  igniting  the  charge  used 
by  Clerk  is  quite  different  from  the  other  flame  methods  already 
described  ;  the  difference  is  necessitated  by  the  greater  rapidity  of 
ignition  in  engines  with  an  impulse  for  ever}'  revolution. 

To  ventilate  the  igniting  port  in  the  Otto  and  Hugon  slides 
requires  time,  which  cannot  be  given  when  the  frequency  of  the 
ignition  approaches  150  to  200  per  minute. 


2 1 6  The  Gas  Engine 

To  meet  this  difficulty  the  author  has  invented  several  methods 
both  flame  and  incandescence,  but  the  one  to  be  described  is 
that  at  present  in  use  in  his  engines  ;  it  is  very  reliable  and  rapid, 
as  many  as  300  ignitions  per  minute  having  been  made  with  it 
experimentally,  or  at  the  rate  of  5  ignitions  per  second. 

A  portion  of  the  explosive  charge  is  allowed  to  pass  from  the 
motor  cylinder  through  a  regulated  passage  to  a  grating  placed  at 
the  end  of  a  cavity  in  the  slide,  and  is  there  ignited  by  a  Bunsen 
flame  ;  the  grating  prevents  the  passage  back  of  the  flame,  and 
the  mixture  burns  in  the  cavity  without  requiring  the  presence  of 
the  external  atmosphere.  At  each  end  of  the  cavity  there  is  a 
port  opening  to  opposite  sides  of  the  valve,  the  one  for  lighting 
the  gases  streaming  from  the  grating,  the  other  for  communicating 
with  the  interior  of  the  cylinder  at  the  proper  time.  The  com- 
munication with  the  cylinder  is  not  made  until  the  outer  port  cuts 
off  from  atmosphere,  and  the  flow  of  the  gases  is  so  regulated  that 
while  this  is  being  done,  the  flame  still  continues  to  be  fed  by 
fresh  supplies.  It  is  evident  that  if  too  great  a  current  be  sent  in, 
the  pressure  will  soon  become  equal  to  that  in  the  cylinder,  and 
then  the  flow  towards  the  cavity  will  cease  and  the  flame  become 
extinguished  ;  this  is  guarded  against  by  proper  proportioning  or 
the  blow  by  the  check  pin.  The  pressure  in  the  cavity  when  its 
port  opens  on  the  cylinder  port  is  still  slightly  less  than  that  in  the 
cylinder,  and  the  gases  from  the  cylinder  enter  and  are  ignited. 
By  using  gas  and  air  already  mixed  in  proper  proportion,  the  ne- 
cessity of  ventilating  is  removed,  and  it  is  made  possible  to  ignite 
at  the  rate  required  by  the  system  of  impulse  at  every  revolution. 
Without  this  it  would  be  almost  impossible  to  get  a  passage  cleared 
out  in  time  to  allow  of  so  frequent  ignition,  by  a  coal  gas  flame 
burning  simply  in  air.  It  was  first  used  by  Clerk  in  an  engine  work- 
ing in  February  1878,  and  has  subsequently  been  used  by  Wittig 
and  Hees  and  by  Robinson  in  the  Tangye  engine.  In  the  form 
here  described  it  was  first  used  by  Clerk  in  November  1880. 

Fig.  78  is  a  sectional  plan  of  the  igniting  slide  and  cover  as 
well  as  the  passage  into  the  combustion  space.  The  valve  i  con- 
tains the  cavity  2,  furnished  at  the  ends  with  the  ports  3  and  4  ;  at 
the  end  3  is  placed  the  grating  5,  communicating  behind  with 
the  explosion  port  6,  by  a  small  hole  7  and  a  gutter  in  the 


Igniting  Arrangements 


217 


valve  face,  showing  at  fig.  78.  A  long  pin  8  screwed  into 
the  end  of  the  slide  controls  the  gases  entering  the  space  behind 
the  grating,  and  if  need  be  can  cut  off  communication  altogether. 
When  the  valve  is  in  the  position  shown  in  the  drawing,  the  mix- 


Valve  in  position  of  flame  lighting  at  external  flame. 
FIG.  78. —  Sectional  Plan,  Clerk  Igniting  Valve. 

ture  is  beginning  to  flow  through  the  grating  into  the  space  2, 
and  is  ignited  by  the  Bunsen  flame  9  lying  up  against  the  valve 
face.  The  Bunsen  flame  lies  so  close  to  the  grating  that  im- 
mediately inflammable  mixture  comes,  it  is  lighted  before  it  can 


218 


The  Gas  Engine 


get  time  to  fill  the  cavity  ;  if  allowed  to  accumulate  in  the  cavity 
before  lighting,  a  slight  explosion  ensues  and  a  disagreeable  report 
is  produced.  The  flame  at  the  grating  burns  in  the  cavity,  dis- 
charging into  the  passage  10,  and  from  thence  to  the  atmo- 
sphere. The  movement  of  the  slide  cuts  off  communication  with 
the  atmosphere,  first  on  the  Bunsen  flame  side,  and  then  on  the 


Internal  flame  exploding  mixture. 
FlG.  79.— Sectional  Plan,  Clerk  Igniting  Valve. 

inside  of  the  valve  ;  very  shortly  after,  the  port  4  opens  on  the  port 
6  leading  to  the  cylinder,  and  the  gases  then  taking  fire  communi- 
cate the  flame  to  the  whole  contents  of  the  compression  space. 
In  fig.  79  the  flame  port  in  the  valve  is  full  open  on  the  explosion 
port  of  the  engine.  The  slide  then  moves  past  the  port  and  back 


Igniting  Arrangements 


219 


to  the  first  position,  where  the  operations  described  are  repeated 
and  igniting  again  occurs. 

This  arrangement  is  very  rapid  in  action,  and  is  capable  of 
igniting  with  the  utmost  regularity  at  a  rate  so  high  as  300  times 
per  minute,  which  is  far  in  excess  of  the  requirements  of  the 
engine.  Fig.  80  shows  the  Bunsen  flame  burning  against  the  face 
of  the  valve,  ready  to  ignite  the  gaseous  mixture. 

C 


—BUNSEN  BURNER 


FlG.  80. — End  Elevation,  Clerk  Igniting  Valve. 

Braytorfs  Flame  Ignition. — The  Brayton  method  of  ignition 
has  already  been  described  shortly  in  the  description  of  the  engine. 
It  is  so  beautiful  and  instructive  that  it  merits  further  discussion. 

The  action  will  be  made  clearer  by  describing  a  well-known 
laboratory  experiment  (fig.  81). 

A  piece  of  wire  gauze,  «,  held  a  few  inches  from  the  Bunsen 
lamp,  £,  the  gas  being  turned  on,  will  prevent  the  flame  when  lit 


22O 


The  Gas  Engine 


above  it  from  pas;  ing  back  through  the  gauze  to  the  burner.  The 
gauze  may  be  moved  through  a  considerable  distance  from  the 
Bunsen  tube  without  extinguishing  the  flame.  The  mixture  of  gas 
and  air  streaming  from  the  Bunsen  passes  through  the  gauze,  and, 
although  igniting  above,  the  heat  is  so  rapidly  conducted  away  by 
the  gauze,  that  the  flame  cannot  pass  through  its  interstices  back 
to  the  lower  side.  If  an  explosive  mixture  be  confined  under  say 
30  Ibs.  per  square  inch  pressure  in  a  vessel,  and  a  pipe  from  it 
(fig.  82)  leads  to  a  pair  of  perforated  plates  with  gauze  between 
them,  a,  then  the  cock  b  being  opened  gently  (the  valve  c  being 
previously  open),  the  mixture  will  stream  through  the  plates  into 


FiG.  81.  —  Bunsen  Flame  burning  above  Gauze. 

the  atmosphere,  and,  if  ignited,  will  burn  at  a  without  passing 
back.  If  the  cock  b  is  opened  suddenly  a  greater  rush  of  flame 
will  occur,  diminishing  again  if  it  is  partly  closed. 

So  long  as  enough  mixture  passes  to  preserve  alive  the  flame 
at  a,  then  any  increased  quantity  passing  from  the  reservoir  will 
be  burned  ;  the  little  flame  increasing  or  diminishing  as  the 
opening  of  the  stop-cock  valve  is  increased  or  diminished. 

The  action  of  the  ignition  in  the  Brayton  engine  is  exactly 
similar.  The  pressure  on  the  flame  side  of  the  grating  is  slightly 
below  that  existing  on  the  other  side  ;  the  stream  of  cold  gases 


Igniting  Arrangements 

entering  the  engine  cylinder  immediately  becomes  flame  on  the 
grating,  and  so  expands,  the  volume  of  flame  being  changed  as 
required  by  the  valve  action  of  the  engine. 

This  method  is  most  successfully  carried  out  in  the  Brayton 
engine.  The  lack  of  economy  is  not  due  to  the  ignition,  but  to 
the  use  of  it  under  unsuitable  circumstances.  Without  doubt  this 


Plan  of  grating. 
FIG.  82. — Brayton  Grating  and  Valve. 

system,  in  a  better  combination,  will  come  largely  into  use  in 
future  and  larger  gas  engines.  It  is  unsuited  for  cold  cylinder 
explosion  engines,  but  admirably  adapted  for  hot  cylinder  com- 
bustion engines  of  the  second  type. 


222 


The  Gas  Engine 


(3)  INCANDESCENCE  METHODS. 

The  ignition  of  explosive  mixtures  by  contact  with  heated 
metallic  surfaces  has  often  been  proposed,  first  by  the  late  Sir 
C.  W.  Siemens,  and  after  him  by  the  American,  Drake.  Dr. 
Siemens,  in  one  of  his  gas  engine  patents,  proposes  to  ignite  the 


FIG.  83.— Sectional  Plan,  Clerk  Incandescent  Platinum  Igniting  Valve. 

mixture  by  passing  it  through  an  iron  tube,  which  is  heated  to  red- 
ness by  a  flame  outside  of  it. 

Drake  constructed  an  engine  in  which  the  ignition  was  effected 
in  a  similar  manner.     The  difficulty  is  found  in  the  rapid  oxidation 


Igniting  Arrangements  223 

of  the  tube,  and  the  consequent  necessity  for  frequent  renewal. 
Frequent  attempts  have  also  been  made  to  heat  a  portion  of  the 
interior  surface  of  the  cylinder,  so  that  at  a  suitable  time  the  mix- 
ture might  be  exposed  to  it  and  fired. 

The  first  arrangement  of  incandescent  ignition  successfully 
applied  to  a  compression  engine  is  the  invention  of  the  author, 
and  is  described  in  his  patent,  No.  3045,  1878.  It  was  used  in  an 
engine  exhibited  at  the  Royal  Agricultural  Society's  Show,  Kilburn, 
in  1879  (July). 

Clerk's  Igniting  Valve. — Fig.  83  is  a  sectional  plan  of  this 
valve  in  position.  Fig.  84  is  a  separate  view  of  the  valve  looking 
upon  the  face,  and  fig.  85  is  the  platinum  cage,  full  size,  taken  out 
of  the  valve. 


FIG.  84.  — Face  of  Valve  with  Platinum  Cage. 


FIG.  85.— Platinum  Cage. 

The  platinum  cage  consists  of  a  box  of  platinum  plate,  with 
numerous  platinum  ribs  running  across  it.  They  are  secured  by 
rivets  running  completely  through,  small  platinum  washers  serving 
to  keep  the  plates  at  equal  distances.  The  valve  receives  this 
cage  in  a  cavity,  and  it  is  tightly  packed  in  its  place  with  asbestos 
and  slate  packing,  a  covering  plate  screwed  down  upon  it  securing 
the  whole  in  position.  To  start  the  engine,  the  reservoir  contain- 
ing gas  and  air  under  pressure  is  opened  ;  the  small  tap,  i,  then 
opened  allows  mixture  to  flow  through  the  diaphragm  2  (made 
like  the  Brayton  grating),  and  the  mixture  is  ignited  at  the 
small  door  3,  which  is  then  closed.  The  flame  flows  through  the 


224 


The  Gas  Engine 


platinum  cage,  heating  up  its  plates  to  a  white  heat  in  a  few 
seconds.  On  opening  the  starting  cock  of  the  engine,  it  moves, 
and  brings  the  igniting  port  4,  on  the  cylinder  port  5,  at  the  same 
time  opening  on  the  port  6,  in  the  cover,  leading  into  the  cavity  7. 
The  mixture  in  the  cylinder  then  rushes  through  the  cage,  becom- 
ing ignited,  and  the  explosion  reaches  the  cylinder  ;  the  cavity  7 

is  so  proportioned  that  each  igni- 
tion sends  a  measured  quantity 
of  flame  through  the  cage  into  it ; 
the  heat  of  the  explosion  at  every 
turn  therefore  supplies  heat  to 
the  platinum.  This  added  heat 
is  sufficient  to  keep  it  at  a  white 
heat.  So  long  as  the  engine  is 
supplied  with  gas  it  gets  an  ig- 
nition at  every  revolution,  and  a 
portion  of  that  heat  goes  to  the 
platinum  to  make  up  for  loss  by 
conduction.  The  heating  flame 
used  in  starting  the  engine  is 
dispensed  with  immediately  on 
starting,  and  the  engine  runs  con- 
tinuously without  outside  flame. 
This  method  is  exceedingly  reli- 
able and  rapid,  but  is  not  suited 
for  the  governing  arrangements 
of  small  engines. 

Siemens'  Tube  Method.—  Fig.  8  6 
is  an  arrangement  of  Siemens' 
method,  as  used  by  Mr.  Atkinson 
in  his  '  Differential '  engine,  ex- 
hibited at  the  Inventions  Exhibi- 
tion. The  wrought  iron  tube  i  is 
FIG.  86.— Hot  Tube  Igniter.  heated  by  the  Bunsen  flame  2, 

the  non-conducting  casing  3  pre- 
venting loss  of  heat ;  the  piston  at  the  proper  time  uncovers  the 
hole  4  into  which  the  tube  is  screwed,  and  the  mixture  entering 
under  pressure  becomes  ignited.  In  other  engines  the  tube  is 


Igniting  Arrangements  225 

caused  to  communicate  with  the  cylinder  by  a  valve.  This  modi- 
fication is  exceedingly  simple  and  works  well ;  care  must  be  taken 
to  avoid  overheating,  or  the  explosion  may  rupture  the  tube.  It 
is  inexpensive  and  easily  renewed  when  disabled  by  oxidation. 

(4)  METHODS  DEPENDING  ON  CATALYTIC  AND 
CHEMICAL  ACTION. 

The  well-known  property  of  spongy  platinum  of  causing  the 
spontaneous  ignition  of  a  stream  of  hydrogen  or  coal  gas  directed 
on  it  in  air,  has  been  proposed  as  a  means  of  ignition  by  Barnett 
(1838).  In  the  arrangement  he  describes,  the  platinum  is  contained 
in  a  little  cup  screwed  into  the  cylinder  cover,  and  the  compression 
of  the  mixture  causes  its  ignition  by  contact. 

Platinum,  however,  soon  loses  this  property,  and  the  action  is 
at  best  too  slow  for  use. 

All  flame  methods  of  course  depend  on  chemical  action,  but 
one  proposal  has  been  made,  to  use  the  property  possessed  by 
phosphorated  hydrogen  of  igniting  spontaneously  in  contact  with 
air.  The  phosphorated  hydrogen  is  conducted  in  small  quantity 
into  the  mixture  to  be  exploded  at  every  revolution,  and  its  com- 
bustion causes  ignition. 

This  proposal  has  never  been  carried  out  in  practice. 

Summary. — To  the  author's  knowledge  no  other  systems 
of  ignition  have  been  proposed  ;  the  flame  methods  are  best 
suited  for  small  gas  engines  and  will  probably  continue  in  use. 
Considerable  improvements  may  still  be  effected  in  ignition  valves, 
and  it  is  possible  that  external  flames  may  be  entirely  done 
away  with  in  future  engines.  It  is  somewha't  humiliating  to  the 
inventor  to  watch  a  powerful  gas  engine  at  work,  developing  say 
30  horses,  and  to  know  that  he  can  at  once  change  the  whole 
and  make  the  engine  powerless  by  blowing  out  the  external  flame. 

A  combination  of  flame  and  incandescence  methods  will  doubt- 
less overcome  this  difficulty,  and  make  the  gas  engine  act  without 
visible  flame  and  without  the  danger  of  extinction  from  draught, 
to  which  the  present  igniting  flames  are  subject. 

It  is  improbable  that  either  the  first  or  fourth  methods  will 
again  find  favour,  the  electric  methods  give  too  much  trouble  and 
are  at  best  uncertain. 


. 
226  TJte  Gas  Engine 


CHAPTER   IX. 

ON    SOME  OTHER    MECHANICAL    DETAILS. 

A  GOOD  working  cycle  and  good  igniting  arrangements  are  the  two 
most  important  factors  in  the  successful  working  of  a  gas  engine, 
but  there  are  other  matters  whose  importance  is  only  secondary 
to  those.  The  governing  gear,  the  oiling  gear,  and  the  starting 
gear,  are  of  the  greatest  importance. 

These  matters  will  now  be  described. 

The  Governing  Gear. — In  the  earlier  gas  engines,  including 
Lenoir  and  Hugon,  the  governing  was  attempted  precisely  as  is 
done  in  the  steam  engine,  the  source  of  power  being  regulated 
by  throttling.  A  centrifugal  governor  acted  upon  a  throttle  valve 
regulating  the  gas  supply,  diminishing  it  when  the  speed  became 
too  great  and  increasing  it  when  the  speed  fell. 

This  was  a  very  bad  and  wasteful  method,  as  the  engineer  will 
at  once  recognise  from  his  knowledge  of  the  properties  of  explosive 
mixtures. 

The  limits  of  change  allowable  in  the  proportions  of  gaseous 
explosive  mixtures  are  very  narrow,  the  gas  present  ranging  from 
\  to  T^  of  the  total  volume.  A  mixture  containing  I  of  its  volume 
of  coal  gas  in  air  has  just  sufficient  oxygen  to  burn  it  and  no 
more  ;  any  further  increase  of  gas  will  pass  away  unburned,  there 
being  insufficient  oxygen  present  for  its  combustion. 

This  is  therefore  the  richest  mixture  which  can  be  used  with 
any  economy. 

A  mixture  of  air  and  gas  containing  TV  of  its  volume  of  gas  is 
in  the  critical  proportion  ;  any  further  dilution,  however  slight,  will 
cause  it  to  lose  inflammability  altogether.  The  governor  may 
act  in  changing  the  proportion  of  gas  and  air  between  those  limits, 
that  is,  the  explosion  may  be  so  reduced  by  dilution  that  it  gives 


On  some  other  Mechanical  Details 


only  naif  the  power  per  impulse  obtainable  with   the  strongest 
mixture. 

Any  further  dilution  causes  the  engine  to  miss  ignition  al- 
together, and  discharge  the  gas  it  has  taken  into  the  exhaust  pipe, 
without  obtaining  any  power  from  it.  If,  therefore,  the  governor 
acts  by  throttling,  the  valve  is  only  closed  enough  to  cause  the 
mixture  to  be  so  weak  as  to  miss  fire ;  as  soon  as  that  point  is 
reached  the  valve  will  be  closed  no  further,  because  at  that  point 
the  speed  of  the  engine  will  cease  to  increase.  Fig.  7,  p.  14,  shows 
the  governor  in  action  upon  a  Lenoir  engine. 


FIG.  87. — Section  showing  Otto  and  Langen  Governor. 

In  modern  compression  engines  the  great  loss  of  gas  occasioned 
by  throttling  is  avoided  by  never  diluting  the  mixture.  Instead  of 
keeping  up  the  same  frequency  of  impulses  but  of  less  power,  as 
done  in  the  steam  engine,  the  gas  is  either  full  on  or  full  off, 
that  is,  the  governing  is  effected  by  diminishing  the  frequency  of 
the  impulses  instead  of  diminishing  their  power. 

In  the  specification  of  the  Otto  engine,  1876  (2081)  the 
governing  is  described  as  being  effected  by  reducing  the  power 
of  the  explosion.  This  is  more  impracticable  in  the  Otto  engine 

Q2 


228 


The  Gas  Engine 


than  in  the  Lenoir,  because,  owing  to  the  dilution  of  the  charge  by 
the  exhaust  gases  or  air,  the  range  of  change  in  mixture  is  smaller. 
The  strongest  mixture  does  not  exceed  i  of  coal  gas  in  8  of  other 
gases. 

Governing— Otto  and  Langen  Engine. — In  this  engine,  in  its 
latest  and  best  form,  the  governing  is  effected  by  missing  impulses. 
When  the  engine  has  received  an  impulse,  the  increase  in  speed 
causes  the  governor  to  move  a  lever  which  disengages  a  pawl  from 
a  ratchet,  and  so  prevents  the  piston  being  raised  and  the  charge 
drawn  into  the  cylinder.  When  the  speed  has  fallen  sufficiently 


FlG.  88. — Otto  and  Langen  Governor,  showing  Pawl  and  Ratchet. 

the  lever  liberates  the  pawl,  and  the  piston  is  then  raised,  taking 
in  the  charge  and  exploding  it.  Fig.  87  is  a  sectional  elevation  of 
the  governing  arrangements.  The  auxiliary  shaft  i  is  driven  from 
the  main  shaft  2  by  the  clutch  3,  but  the  crank  4  and  shaft  i  re- 
ceive motion  from  2  only  by  means  of  the  pawl  5  falling  into  the 
ratchet  3  ;  so  long  as  the  governor  lever  7  remains  in  the  position 
shown,  the  pawl  is  kept  from  engaging  and  the  piston  and  valve 
remain  at  rest ;  so  soon  as  the  governor  lever  7  liberates 
the  pawl,  then  it  falls  into  the  ratchet  wheel  by  a  spring  and  the 


On  some  otJier  Mechanical  Details  229 

auxiliary  shaft  receives  one  turn  ;  the  crank  4,  connected  to  the 
lever  8  (fig.  88),  lifts  the  rack,  and  the  piston  takes  in  its  charge  ; 
at  the  same  time  the  valve  opens  to  gas  and  air,  then,  when 
the  piston  is  full  up,  brings  on  the  igniting  flame.  The  ex- 
plosion occurs  and  shoots  up  the  piston,  which  on  its  down  stroke 
accelerates  the  motion  of  the  power  shaft,  and  if  the  limit  of  speed 
is  exceeded,  the  governor  lever  again  interposes  and  prevents  the 
charge  and  explosion  till  the  speed  falls. 

When  running  without  any  load,  the  two  horse  engine  tested 
at  Manchester  by  Clerk  required  only  6  ignitions  per  minute,  con- 
suming, including  side  lights,  about  25  cubic  feet  per  hour.  The 
shaft  therefore  ran  as  many  as  15  revolutions  merely  by  the  power 
stored  in  the  fly-wheels. 

The  governing  is  effective  but  irregular. 

Governing — Otto  Engine. — The  speed  of  the  Otto  compression 
engine  is  governed  by  diminishing  the  number  of  impulses  given 
to  the  crank  ;  whenever  the  normal  rate  is  exceeded,  the  governor 
so  acts  that  the  gas  supply  is  completely  cut  off  for  one  or  more 
strokes  of  the  engine,  no  impulse  being  given  till  it  falls  again. 

One  arrangement  very  commonly  in  use  is  shown  at  fig.  89. 

The  cam  i  upon  the  auxiliary  shaft  2  is  arranged  to  strike  the 
wheel  3  upon  the  lever  4,  opening  the  gas  valve  5  at  the  begin- 
ning of  the  stroke  and  keeping  it  open  till  the  end  of  the  stroke 
of  the  piston ;  the  gas  passes  from  the  gas  valve  by  a  passage  to  the 
holes  in  the  slide,  when  it  streams  into  the  air  current  entering  the 
engine  by  the  admission  port.  Whenever  the  speed  becomes  high 
enough,  the  governor  6  by  the  lever  7  shifts  the  position  of  the  cam 
i  upon  its  shaft,  so  that  the  wheel  3  does  not  strike  it;  the  gas  valve 
5  therefore  remains  shut  for  that  stroke,  and  the  piston  draws  air 
alone  into  the  cylinder.  When  the  piston  returns  and  compresses 
the  charge,  the  igniting  flame  enters  as  usual,  but  there  being 
no  explosive  mixture  there,  the  piston  moves  out  again  without 
impulse,  expanding  and  discharging,  charging  and  compressing  an 
uninflammable  charge,  till  the  reduction  of  speed  calls  again  for  an 
impulse  ;  the  first  ignition  after  the  engine  has  made  several  re- 
volutions without  gas  is  always  more  powerful  than  the  normal 
one,  because  no  exhaust  gases  being  there  the  charge  mixes  in  the 
space  with  pure  air  and  is  not  heated  previous  to  explosion. 


230 


The  Gas  Engine 


The  arrangement  in  different  Otto  engines  varies  from  this,  but 
the  principle  is  always  the  same. 

Fig.  90  is  a  recent  and  very  clever  governing  arrangement  as 
used  in  the  smaller  Otto  engines. 


FlG.  89. — Otto  Governor  and  Connecting  Gear. 


The  ordinary  governor  is  entirely  dispensed  with,  and  the  valve 
itself  carries  a  pendulum  which  governs. 

The  pendulum  i,  hanging  from  the  pin  2  in  the  slide  valve  3, 
carries  the  long  steel  blade  4,  which  usually  strikes  the  stem  5,  and 
opens  the  gas  valve  at  the  same  time  as  the  slide  opens  to  the  air. 
Whenever  the  speed  is  exceeded,  however,  the  motion  of  the  valve 


On  some  other  Mechanical  Details 


231 


in  the  direction  of  the  arrow,  exceeding  a  certain  rate,  the  pendu- 
lum i  is  left  behind  and  depresses  the  steel  blade  4,  which  therefore 
misses  the  gas  valve  stem  and  for  that  revolution  no  gas  enters. 
So  long  as  the  speed  is  sufficient  to  swing  back  the  pendulum  no 
gas  enters  ;  as  soon  as  it  is  insufficient  to  cause  the  pendulum  to 
leave  its  resting  position  against  the  valve,  then  gas  is  admitted. 

As  the  pressure  of  the  edge  of  the  steel  plate  upon  the  valve 
stem  is  in  direct  line  with  the  centre  of  the  pin  upon  which  the 
pendulum  hangs,  there  is  no  tendency  to  move  it,  that  is,  the 
governor  does  not  furnish  the  power  to  open  the  gas  valve.  In 
all  the  Otto  governing  arrangements  this  principle  is  adhered  to ;  the 


FIG.  90. — Otto  Pendulum  Governor. 

governor  never  furnishes  the  power  to  move  the  gas  valve,  but 
only  signals  to  the  engine  the  proper  time  to  give  the  motion, 
the  motion  being  always  taken  from  the  engine  itself. 

In  electric  light  engines,  which  must  give  the  impulse  for  every 
two  revolutions  with  some  change  of  power,  the  gear  is  modified  ; 
instead  of  complete  cut-off  as  first  described,  the  cam  upon  the 
shaft  is  made  in  several  steps,  so  that  the  wheel  upon  the  gas  lever 
is  shifted  from  one  to  another  as  shown  in  fig.  91.  where  i  is  the 
gas  cam,  and  2  is  the  wheel  upon  the  gas  lever.  Those  steps  are 
made  to  diminish  supply  of  gas  as  much  as  possible  without  miss- 
ing ignition,  so  that  within  narrow  limits  of  changing  load,  the 


232 


The  Gas  Engine 


engine  may  retain  its  frequency  of  impulse.  Whenever  this 
range  of  permissible  variation  is  exceeded,  the  wheel  slips  entirely 
off  the  cam,  and  the  engine  then  governs  in  the  ordinary  manner. 


FIG.  91.  — Otto  Electric  Light  Governor. 

Governing — Brayton  and  Simon. — In  the  Brayton  gas  engine 
the  governing  was  effected  precisely  as  in  the  best  steam  engines, 
by  varying  the  point  of  cut-off.  The  entering  flame  was  cut  off, 
sooner  or  later,  as  determined  by  the  governor  of  the  engine  ;  and 
the  admission  of  gas  and  air  to  the  pump  was  simultaneously 
regulated,  the  amount  entering  being  diminished  to  keep  the 
pressure  in  the  reservoir  constant.  The  diagram,  fig.  45,  p.  158, 
shows  that  the  variable  cut-off  acted  well. 

Fig.  92  shows  the  governor  of  the  petroleum  engine. 

The  cam  i,  which  opens  the  admission  air  valve  on  the  motor 
cylinder,  is  made  tapering,  so  that  the  point  of  cut-off  becomes 
earlier  and  earlier  as  it  slides  in  the  direction  of  the  arrow. 

The  supply  cf  air  was  thus  diminished.  In  this  engine  the 
supply  of  petroleum  could  only  be  diminished  by  hand,  two  screws 
on  the  oil  pump,  when  screwed  upwards,  altering  the  connecting 


On  some  other  Mechanical  Details  233 

rod  between  the  plunger  and  the  eccentric,  giving  more  or  less 
free  movement,  and  thereby  diminishing  the  throw  of  the  pump. 

The  air  supply  to  the  engine  was  not  diminished,  so  that  the 
pressure  in  the  reservoir  increased,  and  was  blown  off  at  a  safety 
valve  placed  upon  the  engine.  This  was  a  wasteful  method. 

The  regularity  of  this  engine  in  running  was  very  great,  being 
far  superior  to  any  of  the  modern  compression  engines.  It  was, 
however,  not  at  all  economical. 

Simon's  engine  presented  no  new  feature  in  its  governing 
arrangements.  They  were  quite  similar  to  Brayton. 


ill 

FIG.  92. — Brayton  Governor. 

Governing — Clerk  Engine. — The  governing  gear  now  used 
upon  this  engine  is  the  design  of  Mr.  G.  H.  Garrett,  Messrs. 
L.  Sterne  &  Co.'s  works'  manager.  It  is  shown  at  figs.  93  and  94. 

It  consists  of  a  gridiron  slide  placed  between  the  upper  and 
lower  lift  valves.  So  long  as  the  engine  is  at  full  power,  the  slide 
i.  fig.  93,  is  moved  by  the  lever  2,  fig.  94,  from  the  ignition  slide 
of  the  engine  already  described,  and  remains  open  during  the 
forward  stroke  of  the  displacer  piston. 

The  charge  of  gas  and  air  therefore  enters  during  the  whole 
stroke,  and  is  sent  into  the  motor  cylinder  to  be  compressed  and 
ignited  at  the  proper  moment  If,  however,  the  load  is  lessened, 
and  the  speed  increases,  and  the  governor  3  acts,  it  moves  the 
lever  4,  which  then  catches  the  lever  2,  and  prevents  the  spring  5 
from  taking  the  slide  i  back  and  opening  it.  The  displacer  then 
discharges  its  contents  into  the  motor  cylinder,  but  on  its  next 
out-stroke,  the  valve  i  being  closed,  it  gets  no  charge  but  the 


234 


The  Gas  Engine 


FIG.  93.— Sections  and  Plan,  Governor  Slide,  Clerk  Engine. 


FIG.  94. — Clerk  Engine  showing  Garrett  Governor  Gear. 


On  some  other  Mechanical  Details 


235 


piston  moves  out,  forming  a  partial  vacuum  behind  it.  The  motor 
cylinder,  therefore,  receives  no  charge  from  the  displacer  cylinder, 
and  the  motor  piston  compresses  and  expands  alternately  the 
burned  gases  behind  it,  while  the  displacer  piston  moves  out  and 
in,  expanding  and  compressing  likewise.  This  goes  on  till  the 
governor  signals  reduction  of  speed,  and  disengages  the  lever  2, 
by  pushing  down  the  lever  4,  so  that  the  spring  5  opens  the  slide 
i,  and  the  engine  gets  a  charge. 

This  method  works  very  well  and  economically  ;  it  is  necessi 
tated   by  the   clearance  space  unavoidable  in  the  Clerk  engine 
between  the  motor  and  displacer  cylinders.     If  gas  were  cut  off  as 
in  the  Otto,  that  space  filled  with  mixture  would  be  lost  every  time 
the  governor  acted. 

Governing — Tangye  Engine. — Messrs.  Tangye's  gas  engine  is 
now  controlled  by  a  very  ingenious   governor,  the  invention  of 


FIG.  95. — Governing— Tangye  Engine. 

Mr.  C.  W.  Pinkney.  It  is  shown  at  fig  95.  The  rod  i  i,  moved 
to  and  fro  by  an  eccentric,  carries  with  it  the  bracket  2,  into 
which  is  fixed  the  pin  3  ;  on  this  pin  the  lever  4  is  swung,  and 
moves  to  and  fro  with  the  bracket ;  the  lever  is  pressed  gently 
downwards  by  the  spring  5,  and  the  lower  part  of  the  lever  is 
formed  into  an  incline  at  6,  so  that  as  it  moves  the  spring  presses 
it  against  the  roller  7.  So  long  as  the  engine  does  not  exceed  its 
proper  speed,  the  lever  4  does  not  rise  above  the  position  shown 


236  The  Gas  Engine 

in  the  figure  when  it  is  moving  in  the  direction  of  the  arrow,  and 
accordingly  its  knife  edge  end  strikes  the  lever  8  and,  acting 
through  the  intermediate  links,  opens  the  gas  valve  9.  The 
engine  gets  its  charge  of  gas  every  time  the  gas  valve  opens.  If 
me  speed  becomes  too  great,  then  the  upward  velocity  given  to 
the  lever  4  by  the  stationary  roller  7  forcing  against  the  incline  is 
such  that  the  knife  edge  lever  4  rises  above  the  end  of  the  lever 
8,  and  the  gas  valve  remains  closed.  When  the  speed  falls 
sufficiently  the  lever  4  again  strikes  the  lever  8  and  opens  the  gas 
valve. 

The  incline  governor  works  well  and  is  exceedingly  sensitive 
to  change  in  speed  :  by  altering  the  compression  upon  the  spring 
5  the  speed  of  the  engine  can  be  varied. 

Oiling  Gear. — In  the  steam  engine  the  comparatively  low 
temperature  of  the  steam  within  the  working  cylinder  and  the 
fact  of  its  condensation  upon  the  walls  and  piston  renders  the 
task  of  lubricating  an  easy  one.  The  lubrication  need  not  be 
absolutely  continuous  and  the  nature  of  the  oil  may  vary  much 
and  no  harm  is  done. 

With  the  gas  engine,  the  intense  flame  filling  the  cylinder  at 
every  stroke  quickly  destroys  the  film  of  oil  with  which  it  is 
covered,  and  necessitates  its  continuous  renewal. 

If  animal  oil  be  used,  its  decomposition  leaves  considerable 
charred  matter,  which  speedily  coats  the  piston  and  cylinder, 
causing  friction  and  danger  of  cutting.  A  good  hydrocarbon,  on 
the  other  hand,  even  when  subjected  to  intense  heat,  decomposes 
into  gases  without  leaving  any  appreciable  amount  of  carbon  : 
mineral  oils  should  therefore  alone  be  used  for  the  cylinder  and 
ignition  slide. 

The  amount  of  oil  required  for  these  parts  is  small  per  day, 
but  it  must  be  regularly  applied  ;  the  burned  film  removed  from 
the  surface  of  the  cylinder  at  every  explosion  must  be  regularly 
replaced  or  abrasion  of  the  surfaces  would  speedily  ensue. 

In  the  Otto  engine  the  oil  required  is  supplied  during  the  whole 
action  of  the  engine  ;  it  commences  with  the  movement  of  the  en- 
gine, continues  so  long  as  it  is  running,  and  stops  when  motion  ceases. 

Fig.  96  shows  the  Otto  oiling  cup,  one  of  which  is  placed,  as 
shown  in  the  drawing,  fig.  97,  at  the  middle  of  the  cylinder  to 


On  some  other  Mechanical  Details 


237 


lubricate  the  piston  and  slide  ;  the  pipes  4  and  5  lead  to  the 
piston  and  slide. 

The  pulley  i  is  driven  slowly  from  the  auxiliary  shaft  by  a 
strap,  and  as  it  rotates  it  carries  the  wire  2  round  on  the  pin  3, 


FIG.  96.— Illustration  of  action  of  Otto  Oiler. 

fig.  96,  alternately  dipping  into  the  oil  and  wiping  it  oft"  to  the 
pin,  from  whence  it  drops  into  the  trough  4  and  runs  by  a  hole 
into  the  tubes.  The  amount  of  oil  so  discharged  can  be  regulated 
by  the  diameter  of  the  wire.  The  oil  flows  along  the  pipes  4  and 


FIG.  97. — Arrangement  of  Otto  Oiler. 

5,  fig.  97,  and  drops  into  holes  at  6  and  7,  the  one  oiling  the  piston 
every  time  the  trunk  comes  forward,  the  other  oiling  the  valve  by 
suitable  gutters. 

The  Clerk  oiling  cup  is  shown  at  fig.  98  ;  it  is  not  automatic. 
The  screw  pin  i  is  set  in  a  position  marked  for  each  cup,  the 


238 


The  Gas  Engine 


motor  cylinder  cup  giving  15  drops  per  minute,  and  the  valve  cup 
5  drops  per  minute. 

In  both  Otto  and  Clerk  engines  the  slide  valves  should  be 
taken  out  and  cleaned  once  a  week.  The  charred  oil  should  be 
carefully  scraped  out  of  the  gas  gutters  and  igniting  ports  ;  the 
piston  also  should  be  drawn  occasionally,  once  in  three  months 
being  sufficient.  The  interior  of  the  cylinder  should  then  be 
cleaned,  especially  the  explosion  space.  The  Otto  exhaust  valve 
should  be  taken  out  every  week  and  cleaned.  The  Clerk  upper 


FIG.  98. -Clerk  Oil  Cup. 

and  lower  lift  valves  require  cleaning  once  ever)7  month  if  the 
engine  is  hard  worked. 

In  working  gas  engines  the  two  points  requiring  attention  are 
oiling  and  cleaning.  Never  run  the  engine,  without  oil,  and  clean 
regularly.  Never  start  without  seeing  that  the  water  circulation 
is  open. 

Starting  Gear. — Till  very  lately,  gas  engines  of  every  power 
were  started  by  manual  labour  ;  in  small  machines  the  inconveni- 
ence is  not  great,  but  with  large  engines  such  as  those  giving 
from  20  to  50  indicated  horses  when  at  full  power,  the  friction 
is  so  considerable  that  difficulties  arise.  It  is  difficult  to  reduce 
friction  so  much  that  a  large  machine  may  be  turned  with  suffi- 
cient velocity  by  a  couple  of  men,  to  get  a  sure  and  easy  start. 

The  Bray  ton  petroleum  engine  was  the  first  to  use  reservoirs 


On  some  other  Mechanical  Details 


239 


for  retaining  sufficient  air  for  starting,  but  they  were  so  faultily 
constructed,  that  leakage  and  loss  were  so  frequent  that  the  appa- 
ratus was  of  little  use.  Many  arrangements  have  been  described 
by  inventors,  but  no  starting  gear  found  its  way  into  public  use 
till  that  invented  by  the  present  author  in  the  end  of  1883.  The 
Clerk  engine  was  the  first  to  use  starting  gear  in  public,  at  the. 


FlG.  99.— Clerk  Starting  Gear. 

beginning  of  1884.  Since  then  over  100  engines  have  been  fitted 
and  are  at  daily  work  with  it. 

The  Otto  engine  speedily  followed  Clerk's  in  the  application 
of  gear,  and  after  them  came  Tangye  and  Atkinson. 

Starting  Gear — Clerk  Engine. — The  starting  gear  used  in  the 
Clerk  engine  is  shown  at  figs.  99  and  100.  Its  action  is  as  follows. 


240 


The  Gas  Engine 


The  flap,  valve  i,  in  the  communicating  pipe  between  the  displacer 
and  motor  cylinders  is  closed,  while  the  engine  is  running,  by  the 
handle  2  ;  the  gases  in  the  displacer  are  thus  prevented  from 
entering  the  motor  cylinder,  and  are  compressed  through  the 
valve  3,  which  is  an  automatic  lift,  into  the  reservoir  4,  by  the  stop- 
valve  5,  which  must  of  course  be  open. 

The  ignition  being  stopped,  the  speed  of  the  engine  falls,  and 
the  flap  is  opened  for  a  few  strokes,  to  allow  the  speed  to  get  up 
again.  It  is  then  closed  again,  this  being  repeated  till  the  reservoir 
4  is  charged  with  a  mixture  of  gas  and  air  at  60  Ibs.  per  sq.  in. 
above  atmosphere.  Five  minutes  gives  ample  time  to  charge 


FIG.  loo.— Clerk  Starting  Valve. 

from  completely  empty  to  60  Ibs.  Three  minutes  suffice  if  the 
charge  has  not  been  completely  taken  from  the  reservoir  during 
the  previous  start.  The  relief  valve  at  6  prevents  charging  above 
60  Ibs.  per  sq.  in.,  the  excess  blowing  into  the  exhaust  pipe. 
When  the  reservoir  is  charged  the  stop  valve  5  is  screwed  down 
and  the  charge  is  retained  in  the  reservoir  till  wanted.  The 
reservoir  is  made  of  steel,  the  sides  being  \  in.  thick  and  the  ends 
f  ;  it  is  welded  throughout,  and  is  tested  before  leaving  the  works 
at  1000  Ibs.  per  sq.  in. 

The  screw  down  valve  5  and  the  joint  where  it  is  screwed 


On  some  other  Mechanical  Details 


241 


into  the  end  form  the  only  joints  for  loss  by  leakage  ;  numerous 
joints  must  be  avoided,  as  it  is  often  necessary  to  leave  the 
reservoir  charged  for  weeks  ;  the  faintest  leakage  would  in  so  long 
a  time  lose  the  contents,  and  so  the  start  would  require  to  be 
made  by  hand. 

The  reservoir  is  so  pressure  tight,  when  made  as  described, 


FIG.  101. — Otto  Starting  Gear. 

that  the  author  has  left  one  standing  for  six  weeks  and  started  the 
engine  with  ease  with  what  remained. 

The  starting  is  effected  as  follows  : 

The  engine  is  placed  in  such  position  that  the  motor  crank  is 
on  the  full  in  centre.  The  displacer  is  therefore  half  forward,  the 
reservoir  stop  valve  is  opened,  the  Bunsen  burner  is  lit,  and  the 
gas  cock  of  the  engine  set  at  the  starting  mark.  The  starting 
handle  7  is  then  moved  in,  opening  the  valve  3,  fig.  TOO,  the  gases 

R 


242  The  Gas  Engine 

entering  press  forward  the  displacer  piston  and  fill  the  compres- 
sion space  of  the  engine,  pressing  forward  the  motor  piston  when 
its  crank  comes  off  the  centre.  The  starting  handle  is  then  let 
go,  and  the  motor  piston  runs  over  its  ports  discharging  the  con- 
tents both  of  motor  and  displacer  to  atmosphere.  The  engine  has 
thus  received  a  double  impulse,  one  in  each  cylinder ;  it  is  enough 
to  bring  round  the  piston,,  compress  the  mixture  and  get  an  igni 
tion.  One  opening  or  at  most  two  openings  of  the  starting  valve  are 
enough.  The  reservoir  contains  enough  to  give  six  successive  starts. 
•  After  starting,  the  reservoir  should  be  again  charged  and  closed 
so  that  it  may  be  ready  when  required. 

The  gear  works  very  well  and  is  easily  handled. 

To  those  accustomed  to  see  gas  engines  started  by  hand,  it  is 
somewhat  astonishing  for  the  first  time  to  watch  a  large  machine 
move  away  at  once  by  a  mere  finger  touch  upon  a  valve. 

Starting  Gear — Otto  Engine. — The  starting  gear  used  in  the 
Otto  engine  is  shown  at  fig.  101.  It  consists  of  the  reservoir  i, 
the  charging  and  starting  valve  2  and  a  stop  valve.  The  charging 
valve  is  loaded  so  that  it  does  not  open  with  a  pressure  less  than 
40  Ibs.  per  square  inch,  as  it  communicates  with  the  compres- 
sion space  4.  It  follows  that  the  compression  of  the  charge  in  the 
cylinder  does  not  lift  it,  but  as  soon  as  the  gases  explode,  the 
pressure  lifts  the  valve,  and  the  reservoir  gets  filled  slowly  with 
burned  gases.  If  the  valve  is  left  open  long  enough  the  pressure 
will  rise  to  within  40  Ibs.  of  the  maximum  explosion  pressure,  that 
is,  about  no  Ibs.  per  square  inch  above  atmosphere.  The  stop 
valve  being  screwed  down,  the  gases  are  retained  ready  to  start 
the  engine  when  wanted.  To  start,  the  stop  valve  at  the  reservoir 
is  opened,  and  the  engine  crank  placed  in  such  position  that  it  is 
off  the  centre  and  on  its  impulse  stroke.  The  gases  then  pass 
through  the  valve  2  into  the  cylinder  4,  and  the  valve  2  closes  at 
the  end  of  the  stroke  actuated  by  the  cam  3.  One  impulse  is 
thus  given  and  is  repeated  at  the  proper  time  by  the  action  of  the 
cam  3  upon  2,  through  the  intermediate  lever.  The  pressure  re- 
quired is  high,  because  only  one  forward  movement  of  the  piston 
is  available  for  every  two  revolutions  of  the  engine.  The  twin 
engine  therefore  starts  more  easily  than  the  ordinary  type  of  Otto. 
This  gear  also  works  well ;  it  was  patented  before  the  Clerk  gear, 
but  was  later  in  being  introduced  into  public  use. 


243 


CHAPTER  X. 

f 

THEORIES   OF    THE   ACTION    OF   THE   GASES    IN    THE   MODERN 
GAS   ENGINE. 

THE  general  principles  developed  in  this  work  explaining  the 
causes  of  the  economy  of  the  modern  gas  engine  were  first  enun- 
ciated by  the  author  in  a  paper  read  before  the  Institution  of 
Civil  Engineers  in  April  I882.1 

He  then  classified  gas  engines  in  three  great  groups  : 

Type  i. — Explosion,  acting  on  piston  connected  to  crank.  (No 
compression.) 

Type  2. — Compression,  with  increase  of  volume  after  ignition, 
but  at  constant  pressure. 

Type  3. — Compression,  with  increase  in  pressure  after  ignition, 
but  at  constant  volume. 

It  was  proved  that  under  comparable  conditions  the  relative 
theoretic  efficiencies  of  the  three  types  were 

Type  i  =  o-2i 
Type  2  =  0-36 
Type  3  =  0-45 

It  was  also  shown  that  in  the  actual  engines  the  real  efficiency 
could  not  be  so  high  as  the  theoretic,  mainly  because  of  the  large 
proportion  of  heat  lost  through  the  sides  of  the  cylinder,  by  the 
exposure  of  the  flame  which  filled  the  cylinder  to  the  comparatively 
cold  enclosing  walls.  A  balance  sheet  was  given  showing  the 
disposal  of  100  heat  units  by  a  compression  engine.  Of  the  100 
heat  units,  17 '83  were  converted  into  indicated  work,  29-28  were 

1     The  Theory  of  the  Gas  Engine,'  by  Dugald  Clerk  :  Minutes,  Institute  Civil 
Engineers,  London.     Paper  No.  1855      April  1082. 

R  2 


244  TJie  Gas  Engine 

discharged  with  the  exhaust  gases,  and  5  2 -89  units  passed  through 
the  sides  of  the  cylinder  into  the  water  jacket. 

The  economy  of  the  Otto  engine  over  its  predecessors,  the 
Lenoir  and  Hugon  engines,  was  clearly  proved  to  be  due  to  the 
fact  of  its  using  compression  previous  to  explosion. 

These  conclusions  were  very  generally  accepted  by  scientific 
and  practical  men  who  had  studied  the  subject,  and  in  February 
1884  the  late  Prof.  Fleeming  Jenkin,  then  Professor  of  Engineering 
at  the  University  of  Edinburgh,  delivered  a  lecture  at  the  Institu- 
tion of  Civil  Engineers  in  London,  on  '  Gas  and  Caloric  Engines.' ' 
He  had  recalculated  the  efficiencies  due  to  compression,  with  the 
result  of  corroborating  the  present  writer's  conclusions.  He 
states  : 

'  If  I  were  to  compress  gas  to  40  Ibs.,  a  pressure  which  is  used 
not  unfrequently,  the  theoretical  efficiency  would  be  45  per  cent. 
We  actually  get  something  like  24  or  23  per  cent. ;  we  know  that 
one-half  of  the  heat  is  taken  away  by  external  cooling.  Thus  we 
find  a  very  close  coincidence  between  the  calculated  efficiency  of 
those  engines  and  that  which  we  actually  obtain,  only  we  throw 
away  about  one-half  of  the  heat  in  keeping  the  cylinder  cool 
enough  to  permit  lubrication.  If  we  compress  to  80  Ibs.  we  have 
a  theoretical  efficiency  of  53  per  cent  If  we  do  not  compress  at 
all,  as  Mr.  Clerk  has  told  you,  we  have  a  theoretical  efficiency  of 
only  21  per  cent,  so  that  we  have  it  in  our  power  to  increase  the 
theoretical  efficiency  very  greatly  by  increasing  the  pressure  of  the 
gas  and  air  before  ignition.  I  have  no  doubt  that  the  great  gain 
of  efficiency  in  the  Clerk  and  Otto  engines  is  really  due  to  the 
fact  of  the  compression  ;  this  being  done  in  a  workmanlike  way 
and  carried  to  a  very  considerable  point.' 

The  advantages  of  compression  could  not  be  stated  with  more 
clearness  and  truth. 

In  the  same  year  there  was  published  in  Paris  an  able  work 
entitled  'Etudes  sur  les  Moteurs  a  Gaz  Tonnant,'  by  Professor  Dr. 
Aime  Witz,  of  Lille,  in  which  the  theoretic  efficiencies  of  the  different 
types  of  cycle  are  calculated  for  a  maximum  temperature  of  ex- 
plosion of  1600°  C,  and  temperature  before  explosion  of  15°  C. 

1  '  Heat  in    ts  Mechanical  Applications  '  :  Institute  Civil  Engineers  Lectures. 
Session  1883-84. 


Theories  of  Action  of  Gases  in  Modern  Gas  Engine    245 

He  adopts  the  same  classification  as  the  present  writer  did  in 
1882,  and  finds  the  efficiencies  : 

Type  i  =  0*28 
Type  2  =  0-38 
Type  3  =  0-44 
which  are  almost  identical  with  the  author's  figures. 

He  also  arrives  at  the  conclusion  that  compression  is  the  great 
source  of  economy  in  the  modern  gas  engine.  At  p.  53  he  says  : 
'  I  find  myself  again  in  agreement  with  Mr.  Dugald  Clerk  when  he 
affirms  that  the  success  of  Otto  is  due  to  compression  alone,  and 
not  to  the  extreme  dilution  of  the  explosive  mixture  in  the  pro- 
ducts of  the  combustion  of  a  precedent  explosion.' 

He  then  proceeds  to  quote  from  the  present  writer's  paper,  and 
adheres  to  the  statement  that — 

'  Without  compression  previous  to  ignition  an  engine  cannot 
be  produced  giving  power  economically  and  with  small  bulk.' 

Compression  previous  to  ignition  gives  two  great  advantages  : 

(1)  A  thermodynamic   advantage   (improved   theory  of  the 
cycle) ; 

(2)  Higher  available  pressures  and  smaller  cooling  surfaces 
— the  joint  result  being  an  economy  in  practice  nearly  fourfold 
that  of  the  old  non-compression  engines. 


MR.  OTTO'S  THEORY. 

Previous  to  1882  the  nature  of  the  improvement  obtained  by 
compression  was  imperfectly  understood,  and  this  notwithstanding 
the  very  clear,  though  qualitative,  statements  of  Schmidt,  Million, 
and  Beau  de  Rochas.  An  erroneous  theory  of  the  cause  of  the 
economy  of  the  Otto  engine  was  widely  circulated  and  gained 
considerable  support. 

It  was  enunciated  in  Mr.  Otto  s  specification  of  1876,  No.  2081, 
and  it  was  and  is  still,  so  far  as  the  author  is  aware,  supported  by 
men  so  distinguished  as  Sir  Frederick  Bramwell,  Dr.  Slaby  of 
Berlin,  Prof.  Dewar  of  the  Royal  Institution,  and  Mr.  John 
Imray. 


246  The  Gas  Engine 

According  to  Mr.  Otto,  all  gas  engines,  previous  to  his  patent 
of  1876,  obtained  their  power  from  the  explosion  of  a  homoge- 
neous charge  of  gas  and  air.  By  the  explosion  excessive  heat  was 
evolved,  and  the  pressures  produced  rapidly  fell  away  ;  the  exces- 
sive heat  was  rapidly  absorbed  by  the  enclosing  cold  walls. 

This  caused  great  loss  and  gave  very  wasteful  engines.  Two 
methods  were  open  to  obtain  better  economy  : 

ist,  by  using  a  very  rapid  expansion,  so  that  the  heat  had  but 
little  time  to  be  dissipated  ; 

2nd,  by  using  slow  combustion  ;  that  is,  by  causing  the  in- 
flammable mixture  to  evolve  its  heat  slowly,  so  that  the  production 
of  excessive  temperatures  and  pressures  was  avoided. 

By  the  first  method  all  the  heat  was  supposed  to  be  evolved  at 
once,  and  a  high  temperature  was  produced  :  by  the  second 
method  the  heat  was  evolved  gradually  so  as  to  give  a  low  temper- 
ature and  pressure  which  was  sustained  throughout  the  stroke, 
and  which  was  advantageously  utilised  by  the  piston  while  moving 
at  a  moderate  speed.  Mr.  Otto  states  that  this  gradual  evolution 
of  heat  may  be  produced  by  stratifying  the  charge  of  gas  and  air. 
Instead  of  using  the  homogeneous  charge  of  Lenoir  and  Hugon, 
Mr.  Otto  uses  a  charge  which  he  states  is  not  homogeneous  but 
heterogeneous.  He  affirms  that  his  invention  lies  in  the  method  or 
process  of  forming  this  stratified  charge  in  a  gas-engine  cylinder, 
and  that,  in  addition  to  the  explosive  mixture,  there  must  be  present 
in  the  cylinder  a  mass  of  inert  gas  which  does  not  burn  but  which 
serves  to  absorb  the  heat  of  the  explosion  and  prevent  the  loss 
which  would  otherwise  occur  by  the  cooling  effect  of  the  cylinder 
walls. 

The  '  inert '  gas  may  be  either  air  alone  which  is  capable  of 
supporting  combustion,  or  the  products  of  combustion  which  are 
incapable  of  supporting  combustion,  or  a  mixture  of  both.  It  is 
not  sufficient  that  a  mere  film  of  this  inert  gas  be  present ;  there 
must  be  what  is  termed  a  '  notable '  quantity. 

Mr.  Otto  proposes  to  form  this  heterogeneous  or  stratified 
charge  by  first  drawing  into  the  cylinder  a  charge  of  air  alone ; 
and  second,  a  charge  of  explosive  mixture,  or  by  leaving  in  the 
cylinder  a  sufficient  quantity  of  the  products  of  a  previous  com- 
bustion to  form  a  '  notable '  quantity  of  inert  diluent. 


Tlieories  of  Action  of  Gases  in  Modern  Gas  Engine    247 

The  compression  space  in  the  Otto  engine  is  supposed  to  con- 
tain a  sufficient  volume  of  burned  gases  to  form  the  inert  diluent, 
so  that  the  whole  stroke  of  the  piston  is  available  for  taking  in  the 
explosive  charge. 

Suppose  the  piston  to  begin  its  charging  stroke  :  the  coal-gas 
and  air  mixture  flows  into  the  cylinder  through  the  inlet  port  and 
mixes  to  some  extent  with  the  inert  gas  already  in  the  space  ;  but 
the  mixing  is  incomplete,  and  at  the  piston  itself  the  charge  is 
supposed  to  consist  entirely  of  exhaust  gases.  So  that,  while 
the  charge  at  the  igniting  port  is  readily  explosive,  that  at  the 
piston  is  not  explosive  at  all,  and  between  the  igniting  port  and 
the  piston  the  composition  of  the  charge  varies  from  point  to 
point. 

This  '  arrangement  of  the  gases '  is  supposed  to  be  retained 
during  compression,  and  exist  at  the  moment  of  explosion.  The 
compression  space  contains  a  '  packed  charge,'  which  consists  of 
an  explosive  mixture  at  the  one  end,  and  between  the  explosive 
mixture  and  the  piston  a  cushion  of  inert  fluid,  which  is  uninflam- 
mable and  serves  the  double  purpose  of  relieving  the  piston  from 
the  shock  of  explosion  and  absorbing  heat  which  would  otherwise 
be  lost  by  conduction. 

By  this  device,  heat  is  gradually  evolved.  The  flame  originated 
in  the  port  burns  at  first  with  great  energy  and  spreads  from  one 
combustible  particle  to  another,  more  and  more  slowly  as  it  ap- 
proaches the  piston,  where  the  particles  are  dispersed  more  and 
more  in  the  inert  gas.  The  mixture  is  so  arranged  that  this  burn- 
ing lasts  throughout  the  whole  stroke,  and  is  complete  very  shortly 
before  the  exhaust  valve  opens. 

The  entire  cylinder  is  never  completely  filled  with  flame,  but 
the  charge  at  one  end  has  burned  out  before  the  flame  arrives  at 
the  other  end. 

Dr.  Slaby  comes  forward  in  support  of  this  hypothesis  in  an 
interesting  report  published  as  an  Appendix  to  Prof.  Fleeming 
Jenkins'  lecture  already  referred  to. 

Dr.  Slaby  states  :  '  The  essence  of  Otto's  invention  consists  in 
a  definite  arrangement  of  the  explosive  gaseous  mixture,  in  con- 
junction with  inert  gas,  so  as  to  suppress  explosion  (and  neverthe- 
less insure  ignition). 


248  The  Gas  Engine 

'  At  the  touch  hole,  where  the  igniting  flame  is  applied,  lies  a 
strong  combustible  mixture  which  ignites  with  certainty.  The 
flame  of  this  strong  charge  enters  the  cylinder  like  a  shot,  and 
during  the  advance  of  the  piston  it  effects  the  combustion  of  the 
farther  layers  of  dispersed  gaseous  mixture,  whilst  the  shock  is 
deadened  by  the  cushion  of  inert  gases  interposed  between  the  com- 
bustible charge  and  the  piston. 

'The  complete  action  takes  place  in  a  cycle  of  four  piston 
strokes.  The  first  serves  for  drawing  in  the  gases  in  their  proper 
arrangement  and  mixture  ;  the  second  compresses  the  charge  ; 
during  the  third  the  gases  are  ignited  and  expand  ;  and  finally, 
by  the  fourth  the  products  of  combustion  are  expelled.  The 
essential  part  of  the  working  is  performed  by  the  first  of  these 
strokes,  by  which  the  charge  is  drawn  in  and  arranged,  first  air, 
then  dilute  combustible  mixture,  and  finally  strong  combustible 
mixture.  This  arrangement  is  obtained  by  the  working  of  the 
admission  slide.  Moreover,  after  discharge  of  the  products  of 
combustion,  a  portion  remains  in  the  clearance  space  of  the 
cylinder,  and  this  constitutes  the  inert  layer  next  the  piston.  By 
this  peculiar  arrangement  of  the  gases,  the  ignition  and  combus- 
tion above  described  are  rendered  possible,  whilst  the  products  of 
previous  combustion  form  a  cushion,  saving  the  piston  from  the 
shock  of  the  explosion  of  the  strongly  combustible  mixture  at  the 
farther  end  of  the  cylinder.' 

Having  stated  the  essence  of  Otto's  invention,  Dr.  Slaby  pro- 
ceeds to  compare  the  Otto  and  Lenoir  indicator  diagrams,  to  show 
that  the  Otto  diagrams  prove  that  the  above  actions  occur  in  the 
engine.  He  finds  that  the  Otto  expansion  line  is  somewhat  above 
the  adiabatic  line,  and  that  the  Lenoir  expansion  line  is  below  it. 
That  is,  the  Otto  diagram  gives  evidence  of  heat  being  added  or 
combustion  proceeding  in  the  cylinder  during  the  whole  expansion 
stroke,  and  the  Lenoir  diagram  gives  evidence  of  loss  of  heat,  not 
gain,  during  a  similar  period.  If  a  mass  of  expanding  gas  traces 
on  the  diagram  the  adiabatic  line,  then  it  appears  as  if  no  loss  of 
heat  occurred  ;  but  as  the  temperature  of  the  flame  filling  the 
cylinder  is  known  to  exceed  1200°  C.,  it  must  be  losing  heat  to 
the  water  jacket.  To  make  the  expansion  line  keep  up  to  the 
adiabatic  a  great  flow  of  heat  into  the  gas  must  be  taking  place, 


Theories  of  Action  of  Gases  in  Modern  Gas  Engine    249 

and  as  the  only  source  of  heat  is  combustion,  it  follows  that  the 
gas  is  burning  during  the  expansion  period. 

Dr.  Slaby  calculates  the  proportion  of  heat  evolved  by  the  ex- 
plosion in  the  Otto  engine  as  55  per  cent.,  leaving  45  per  cent  to 
be  evolved  during  expansion. 

This  he  states  is  due  to  the  portion  of  the  charge  which  con- 
tinues to  burn  after  the  explosion. 

The  curve  differs  from  Lenoir's  in  this,  that  while  in  Lenoir's 
engine  all  the  heat  is  evoked  at  the  moment  of  explosion,  leaving 
none  to  be  evolved  during  expansion,  in  Otto's  only  a  part  is 
evolved  at  first,  and  the  reserved  portion  keeps  up  the  temperature 
during  expansion. 

He  concludes  from  his  experiments  that  the  action  of  the 
Otto  engine  is  truly  as  Mr.  Otto  states  in  his  specification — 
explosion  is  suppressed  and  a  slow  evolution  of  heat  is  obtained, 
and  this  slow  evolution  of  heat  is  the  result  of  the  invention  and 
the  cause  of  the  economy  of  the  engine. 

In  addition  to  this  indirect  proof,  experiments  have  been  made 
at  Deutz  and  elsewhere  to  show  directly  that  stratification  has  a 
real  existence  in  the  Otto  engine. 

An  Otto  engine  was  constructed,  specially  fitted  with  two  igni 
ting  valves  ;  one  valve  was  placed  on  the  side  of  the  cylinder  at 
the  end  of  the  explosion  space  next  the  piston,  so  that  it  could 
ignite  the  gases  at  the  piston  ;  the  other  valve  was  the  usual  one 
at  the  end  of  the  cylinder,  igniting  the  gases  in  the  admission 
port. 

Experiments  were  made  to  discover  if  the  side  valve  would 
fire  the  mixture  at  the  piston  ;  it  was  found  that  it  did  so.  Con- 
secutive ignitions  were  obtained  there. 

Diagrams  were  taken  for  comparison,  with  the  end  and  the 
side  valves  in  alternate  action,  care  being  taken  to  keep  the  charge 
in  the  same  proportions  during  the  trials.  It  was  found  that 
although  the  side  valve  ignited  as  regularly  as  the  end  valve,  yet 
the  diagrams  were  different.  Instead  of  the  usual  rapid  ascending 
explosion  line,  the  explosion  took  place  more  slowly,  and  the 
maximum  pressure  was  not  attained  till  late  in  the  stroke. 

The  ignitions  were  slower  from  the  side  valve  than  from  the 
end  valve.  If  an  uninflammable  cushion,  such  as  Dr.  Slaby  so 


250  The  Gas  Engine 

clearly  describes,  existed  at  the  piston,  one  would  expect  that  the 
side  valve  would  fail  entirely,  but  it  ignited  quite  regularly  although 
more  slowly  than  the  end  valve. 

This  experiment  is  considered  to  prove  stratification. 

To  make  stratification  visible  to  the  eye,  a  small  glass  mode 
was  constructed.  It  consisted  of  a  glass  cylinder  of  about  ii  ins. 
internal  diameter,  containing  a  tightly  packed  piston  connected  to 
a  crank  ;  the  stroke  was  about  6  ins.  ;  when  full  back,  the  piston 
left  a  considerable  space  to  repiesent  the  explosion  space.  A 
brass  cover  was  fitted  to  the  end  of  the  tube,  and  in  it  was  bored 
a  hole  of  about  f  in.  diameter,  representing  the  admission  port ; 
in  this  hole  was  screwed  a  pet  cock  to  which  a  cigarette  was 
affixed. 

On  lighting  the  cigarette  and  then  moving  the  piston  forward 
by  the  crank,  it  was  seen  that  the  smoke  of  the  cigarette  which 
passed  in  did  not  completely  fill  the  cylinder ;  the  smoke  slowly 
oozed  in  and  left  a  large  clear  space  between  it  and  the  piston. 
The  smoke  was  supposed  to  represent  the  charge  of  gas  and  air 
rushing  in,  and  the  clear  air  behind  the  piston  the  cushion  which 
was  said  to  exist  in  the  Otto  engine.  It  was  supposed  that  in  the 
glass  cylinder  was  repeated  on  a  small  scale  the  action  of  the  gases 
occurring  on  a  larger  scale  in  the  Otto  engine.  In  a  recent  paper 
in  a  German  engineering  journal,  Dr.  Slaby  recounts  this  experi- 
ment, and  lays  great  weight  upon  it.  He  considers  that  it  un- 
doubtedly proves  the  truth  of  the  Otto  theory. 

In  discussion  Mr.  John  Imray  concisely  states  the  Otto 
position  as  follows  : 

'The  change  which  Mr.  Otto  had  introduced,  and  which 
rendered  the  engine  a  success  was  this  :  that  instead  of  burning 
in  the  cylinder  an  explosive  mixture  of  gas  and  air,  he  burned  it 
in  company  with,  and  arranged  in  a  certain  way  in  respect  of,  a 
large  volume  of  incombustible  gas  which  was  heated  by  it,  and 
which  diminished  the  speed  of  combustion.' 

And  Mr.  Bousfield  states  it  in  similar  terms  : 

'In  the  Otto  gas  engine  the  charge,  varied  from  a  charge 
which  was  an  explosive  mixture  at  the  point  of  ignition  to  a  charge 
which  was  merely  an  inert  fluid  near  the  piston.  When  ignition 
took  place,  there  was  an  explosion  close  to  the  point  of  ignition 


Theories  of  Action  of  Gases  in  Modern  Gas  Engine    251 

that  was  gradually  communicated  throughout  the  mass  of  the 
cylinder.  As  the  ignition  got  further  away  from  the  primary  point 
of  ignition,  the  rate  of  transmission  became  slower,  and  if  the 
engine  were  not  worked  too  fast  the  ignition  should  gradually 
catch  up  the  piston  during  its  travel,  all  the  combustible  gas  being 
thus  consumed.  When  the  engine  was  worked  properly  the  rate 
of  ignition  and  the  speed  of  the  engine  ought  to  be  so  timed  that 
the  whole  of  the  gaseous  contents  of  the  cylinder  should  have 
been  burned  out  and  have  done  their  work  some  little  time  before 
the  exhaust  took  place,  so  that  their  full  effect  could  be  seen  in 
the  working  of  the  engine.  This  was  the  theory  of  the  Otto  engine.' 
From  these  quotations  it  will  be  seen  that  Mr.  Otto's  supporters 
agree  that  Mr.  Otto  has  invented  a  means  of  suppressing  explosion, 
and  substituting  for  explosion  a  regulated  combustion,  and  that 
this  process  is  the  cause  of  the  economy  of  the  engine.  They 
are  agreed  that  he  has  succeeded  in  preventing  explosion,  and 
that  he  does  this  by  arranging  or  stratifying  the  charge  which  is 
to  be  used.  They  consider  that  engines  previous  to  Mr.  Otto's 
were  wasteful  because  they  used  a  homogeneous  and  therefore 
explosive  charge,  and  that  Mr.  Otto's  engine  is  economical  be- 
cause it  uses  a  heterogeneous  or  stratified  charge,  which  is  con- 
sequently non-explosive. 

Discussion  of  Mr.  Otto's  Theory. 

The  primary  fallacy  of  Mr.  Otto's  theory  lies  in  the  assumption 
that  previous  engines  were  more  explosive  than  his,  and  that  in 
previous  engines  all  the  heat  was  evolved  at  once  :  as  a  plain 
matter  of  fact  this  is  incorrect.  In  the  Lenoir  and  Hugon  engines, 
as  in  all  explosive  engines,  little  more  than  one-half  of  the  total 
heat  is  evolved  by  the  explosion,  and  the  portion  reserved  is  evolved 
during  the  stroke  of  the  engine. 

The  following  test  of  a  Lenoir  engine,  made  by  the  author  in 
London,  very  clearly  shows  the  suppression  of  heat  at  first  : 

Lenoir  engine  rated  at  one  horse  power. 
Cylinder  7^  inches  diameter;  stroke  nf  inches. 
Average  revolutions  during  test,  85  per  minute. 
Gas  consumed  in  one  hour,  86  cubic  feet. 


252  The  Gas  Engine 

With  full  load,  indicated  horse  power,  1-17    (average   of  9 

diagrams). 
Gas  consumed  per  indicated  horse  power  per  hour,  73-5  cubic 

feet 

Maximum  temperatures  of  explosion,  1100°  to  1200°  C. 
Mixture  in  engine  i  vol.  coal  gas,  12^5  vols.  of  air  and  other 

gases. 
Heat  evolved  by  explosion,  60  per  cent,  of  total  heat. 

The  proportion  of  the  mixture  was  calculated  from  the  points  of 
cut-off  on  the  diagram,  and  after  making  allowance  for  the  volume 
of  burned  gases  in  the  clearances  of  the  engine.  It  will  be 
observed  that  only  60  per  cent,  of  the  gas  is  burned  at  first,  leaving 
40  per  cent,  to  be  burned  during  the  stroke,  and  also  that  the 
temperature  of  the  explosion  never  exceeds  1200°  C.  Now  in  the 
Otto  engine,  according  to  Thurston,  60  per  cent,  of  the  heat  is 
evolved  at  explosion,  and  40  afterwards,  and  the  usual  maximum 
temperature  is  about  1600°  C.  So  that,  so  far  as  the  slowness  of 
the  explosion  is  concerned,  there  is  no  difference,  and  in  the  in- 
tensity of  the  temperature  produced,  the  Otto  exceeds  the  Lenoir. 

It  is  difficult  to  understand  how  Dr.  Slaby  could  fall  into  so 
obvious  an  error  as  he  did,  and  suppose  that  more  heat  was  kept 
back  in  the  case  of  the  Otto  explosion.  At  the  time  he  wrote  his 
report,  accounts  of  Hirn's,  Bunsen's,  and  Mallard's  experiments 
on  explosion  were  in  existence,  all  of  them  agreeing  on  the  fact 
of  a  large  suppression  of  heat  at  the  maximum  temperature  of  the 
explosion,  although  differing  in  the  explanation  of  the  fact. 

Him  even  stated  that  in  the  Lenoir  engine  the  pressures  fell 
far  short  of  what  should  be,  if  all  the  heat  were  evolved  at  once. 
Yet  Dr.  Slaby,  in  the  presence  of  all  this  definite  and  carefully 
ascertained  knowledge,  is  astonished  when  he  finds  only  55  per 
cent,  of  the  total  heat  evolved  by  the  explosion  in  the  Otto  engine, 
and  the  only  explanation  which  occurs  to  him  is  that  of  stratifica- 
tion. 

If  stratification  exists  at  all  in  the  engine,  then  it  produces  no 
measurable  change  in  the  explosion  ;  it  neither  retards  the  evolu- 
tion of  heat,  nor  does  it  moderate  the  temperature. 

The  explosion  and  expansion  curves  are  precisely  what  they 
would  have  been  with  a  homogeneous  charge. 


Theories  of  Action  of  Gases  in  Modern  Gas  Engine    253 

The  mere  fact  that  heat  is  suppressed  in  the  Otto  explosion 
proves  nothing,  because  a  precisely  equivalent  amount  of  heat  is 
suppressed  in  all  gaseous  explosions,  and  Dr.  Slaby's  contention, 
based  upon  the  supposed  peculiarity  of  the  Otto,  falls  to  the 
ground. 

Dr.  Slaby  has  been  led  into  error  by  the  fact  that  the  expansion 
line  of  the  Lenoir  diagram  falls  below  the  adiabatic,  while  the 
expansion  line  of  the  Otto  diagram  remains  slightly  above  it  or 
upon  it.  He  assumes  that  in  the  Lenoir  no  heat  is  being  added 
during  expansion,  whereas  just  as  much  heat  is  being  added,  or 
just  as  much  combustion  is  proceeding  during  the  Lenoir  stroke, 
only  the  cooling  of  the  cylinder  walls  is  greater,  and  the  heat  is 
abstracted  so  rapidly  that  the  line  falls  below  the  adiabatic.  This 
is  due  to  two  causes,  (i)  the  greater  proportional  cooling  surface 
exposed  by  the  Lenoir  engine,  and  (2)  a  longer  time  of  exposure. 
The  absence  of  compression  and  the  slow  piston  speed  makes  the 
loss  greater. 

Although  quite  as  much  heat  is  evolved  during  the  stroke,  it 
is  overpowered  by  the  greater  cooling,  and  the  line  falls  under  the 
adiabatic.  This  fall  is  evidence  of  greater  cooling,  not  of  less 
evolution  of  heat. 

In  a  recent  paper,1  'Die  Verbrennung  in  der  Gasmaschine, 
Professor  Schottler  makes  this  explanation  of  the  difference  be- 
tween the  lines,  and  states  that  'Whether  stratification  exists  or 
does  not  exist  in  the  Otto  engine  it  is  unnecessary,  and  is  not  the 
cause  of  the  slow  falling  of  the  expansion  line.'  In  all  crucial 
points  the  Otto  theory  breaks  down,  as  proved  by  diagrams  taken 
from  his  engine. 

The  explosion  is  not  suppressed  ;  the  maximum  temperatures 
produced  are  not  lower  than  those  previously  used  ;  the  mixture 
used  is  not  more  diluted  than  in  the  previous  engines,  and  the  in- 
tensity of  the  pressures,  as  well  as  the  rate  of  their  application,  is 
greater. 

The  mixture  in  the  engine  from  Slaby's  figures  is  i  vol.  coal 
gas  to  10 -5  vols.  of  other  gases,  and  from  Thurston's  figures  i 
vol.  coal  gas  to  9'!  vols.  of  other  gases,  while  Lenoir  often  used 
i  vol.  gas  to  12  of  air. 

1  Zeitschrift  des  Vereines  deutscher  Ingenieure.     Band  xxx.,  Seite  209. 


254  The  Gas  Engine 

The  engine  instead  of  using  a  less  explosive  power  than  the 
Lenoir  engine  uses  one  more  intensely  explosive. 

The  effect  of  the  reduction  of  cooling  surface  and  increase  of 
piston  velocity  is  to  diminish  the  loss  of  heat  to  the  cylinder  walls, 
and  the  slowly  descending  line  is  not  the  cause  of  the  economy, 
but  is  the  effect  and  evidence  of  it 

Stratification. — The  inquiry  into  the  existence  or  non-existence 
of  stratification  in  the  cylinder  has  no  practical  bearing  on  the 
question  of  economy,  as  the  explosion  curves  act  precisely  as  they 
would  with  homogeneous  mixtures.  Scientifically,  however,  the 
question  is  interesting  and  will  be  shortly  considered. 

The  evidence  which  it  is  considered  proves  its  existence  in  the 
Otto  engine  is  in  the  author's  opinion  most  unsatisfactory.  Dr. 
Slaby  distinctly  asserts  the  existence  of  an  inert  stratum  next  the 
piston,  'interposed  between  the  combustible  charge  and  the  piston,' 
and  Mr.  Imray  speaks  of  the  'arrangement  of  the  charge  in  respect 
of  a  large  volume  of  incombustible  gases/  and  Mr.  Bousfield  of 
'a  charge  which  was  merely  an  inert  fluid  next  the  piston.'  Yet 
all  the  evidence  in  support  of  these  positive  assertions  is  given  by 
one  experiment  made  with  an  Otto  engine,  and  one  with  a  small 
glass  model.  The  evidence  given  by  the  experiment  on  the  engine 
itself,  in  the  author's  opinion,  disproves  stratification  in  the  Otto 
sense  altogether.  If  the  inert  stratum  next  to  the  piston  had  any 
real  existence,  then  the  side  igniting  valve  in  the  experiment  made 
by  Mr.  Otto,  should  not  have  ignited  the  mixture  at  all.  The  fact 
that  it  did  ignite  regularly  and  consecutively,  proved  most  dis- 
tinctly that  the  gas  next  the  piston  was  not  inert  but  was  explosive, 
and  being  explosive  in  itself  it  could  not  act  as  a  cushion  to 
absorb  heat  or  shock.  That  experiment  alone  settles  the  ques- 
tion, and  proves  at  once  the  visionary  nature  of  the  cushion  of 
inert  gas  next  the  piston. 

The  fact  that  the  ignitions  were  slower  than  those  from  the  end 
slide  does  not  get  rid  of  the  fact  that  ignition  did  take  place,  and 
to  those  who  understand  the  sensitive  nature  of  any  igniting  valve, 
it  will  not  be  difficult  to  comprehend  how  small  a  difference  in 
adjustment  will  cause  late  and  slow  ignitions.  At  the  very  utmost 
the  experiment  points  to  a  small  difference  in  the  dilution  of  the 
explosive  mixture  at  the  piston  and  that  at  the  end  port. 


TJieories  of  Action  of  Gases  in  Modern  Gas  Engine    255 

Experiments  made  by  the  author  also  prove  that  the  mixture 
in  the  Otto  cylinder  is  present  in  explosive  proportions  close  up  to 
the  piston.  The  piston  of  a  3^  HP  Otto  engine  was  bored  and 
fitted  with  a  screw  plug,  which  carried  a  small  spiral  of  platinum 
wire  in  electrical  connection  with  a  battery  ;  the  platinum  spiral 
projected  from  the  inner  surface  of  the  piston  by  a  quarter 
of  an  inch.  When  the  engine  was  running  in  the  usual  way,  the 
wire  was  made  incandescent  by  the  battery  and  the  external  light 
was  put  out.  It  was  proved  that  by  a  little  care  in  getting  the 
platinum  to  a  certain  temperature,  the  engine  worked  as  usual, 
igniting  regularly  and  consecutively.  The  spiral  was  made  just 
hot  enough  to  ignite  when  compression  was  complete,  but  not  hot 
enough  to  ignite  before  compressing.  If  an  incombustible  stratum 
had  existed  even  so  close  to  the  piston  as  £  in.  then  the  wire 
should  never  have  been  able  to  ignite  the  charge  at  all.  If  the 
wire  was  made  too  hot,  then  ignition  often  took  place  while  the 
charge  was  still  entering,  proving  that  no  stratification  existed  even 
while  the  charge  was  incomplete.  A  little  consideration  of  the 
arrangement  of  the  Otto  engine  will  show  that  stratification  can- 
not have  any  existence  in  it.  The  end  of  the  combustion  space  is 
usually  flat,  and  sometimes  the  admission  port  projects  slightly 
into  it ;  the  area  of  the  admission  port  is  about  ^  of  the  piston 
area  ;  accordingly  the  entering  gases  flow  into  the  cylinder  at  a 
velocity  thirty  times  the  piston  velocity,  or  at  the  Otto  piston 
speed,  about  120  miles  an  hour. 

Great  commotion  inevitably  occurs  ;  the  entering  jet  projects 
itself  through  the  gases  right  up  against  the  piston,  and  then  re- 
turns eddying  and  whirling  till  it  mixes  thoroughly  with  whatever 
may  be  in  the  cylinder.  The  mixture  becomes  practically  homo- 
geneous even  before  compression  commences. 

Experiments  made  by  Dr.  John  Hopkinson  and  the  author  on 
full  size  glass  models  of  the  Otto  cylinder  show  this  mixing  action 
very  beautifully.  A  3^  HP  Otto  cylinder  was  copied  in  every 
proportion  in  glass,  and  the  valve  was  so  arranged  that  it  passed  a 
charge  of  smoke  at  the  proper  time.  The  piston  was  placed  at 
the  end  of  its  stroke,  leaving  the  compression  space  filled  with  air. 
When  pulled  forward  the  valve  opened  to  a  chamber  filled  with 
smoke,  and  the  smoke  rushed  through  the  port,  projected  right 


256  The  Gas  Engine 

through  the  air  in  the  space,  struck  the  piston,  and  filled  the 
cylinder  uniformly,  much  faster  than  the  eye  could  follow  it.  It 
mixed  instantaneously  with  the  air  in  the  cylinder  without  evincing 
the  slightest  tendency  to  arrange  itself  in  the  manner  imagined 
by  Mr.  Otto.  Mr.  Otto's  experiment  with  a  cigarette  and  glass 
cylinder  does  not,  in  the  most  remote  degree,  imitate  the  condi- 
tions occurring  in  his  engine;  the  proportions  are  quite  wrong. 
The  model  is  much  too  small,  and  the  glass  cylinder  is  too  long 
in  proportion  to  its  diameter ;  then  the  gases  are  so  badly  throttled 
by  passing  through  the  cigarette,  that  when  the  piston  is  moved 
forward  it  leaves  a  partial  vacuum  behind  it,  and  only  a  little 
smoke  enters,  not  nearly  enough  to  follow  up  the  piston,  but 
only  sufficient  to  ooze  into  the  back  of  the  cylinder  while  the 
piston  moves  forward  and  expands  the  air  which  is  already  in  the 
cylinder.  Ic  was  easy  for  Mr.  Otto  to  have  copied  his  cylinder 
and  valve  full  size  and  imitated  precisely  the  conditions  existing  in 
his  engines. 

Had  he  done  this  he  would  have  proved  complete  mixing  in- 
stead of  stratification.  Why  did  he  refrain  from  doing  this  ?  The 
question  at  issue  is  not,  Can  stratification  be  obtained  by  a  speci 
ally  devised  form  of  apparatus — no  one  doubts  that  it  can— but, 
Does  stratification  exist  in  the  Otto  engine  ?  If  it  does  not 
exist  in  the  Otto  engine  then  it  is  perfectly  plain  that  it  cannot  be 
the  cause  of  the  economy  of  the  motor,  and  it  is  quite  certain  that 
it  cannot  exist  in  the  Otto  engine.  Prof.  Schottler,  in  the  paper 
already  referred  to,  also  arrives  at  the  conclusion  that  stratification 
has  no  existence  in  the  Otto  engine,  and  that  Mr.  Otto's  small 
glass  model  does  not  truly  represent  the  actions  occurring  in  the 
engine. 

In  all  gas  engines,  when  the  charge  enters  the  cylinder  through 
a  port  the  residual  gases  in  the  port  are  swept  into  the  cylinder, 
and  while  the  port  itself  is  filled  with  gas  and  air  mixture,  free 
from  admixture  with  residual  gases,  the  cylinder  contains  the  gas 
and  air  mixture  diluted  with  whatever  residual  gases  exist  in 
the  engine  which  have  not  been  expelled  by  the  piston.  The 
mixture  in  the  port  is  accordingly  stronger  and  more  inflammable 
than  the  mixture  in  the  cylinder. 

In  the  Lenoir  and  Hugon  engines  this  occurred  to  a  marked 


Theories  of  Action  of  Gases  in  Modern  Gas  Engine    257 

extent ;  in  the  Hugon  engine  as  much  as  30  per  cent,  of  the  whole 
charge  consisted  of  residual  gases,  and  the  charge  in  the  cylinder 
was  considerably  more  dilute  than  that  in  the  admission  port.  In 
the  Otto  engine  this  also  occurs,  but  it  is  not  stratification,  and  it 
is  not  a  new  invention  ;  the  cylinder  is  filled  with  explosive  mix- 
ture more  dilute  than  that  in  the  ignition  port,  but  still  explosive 
throughout. 


CAUSES  OF  THE  SUPPRESSION  OF  HEAT  AT  MAXIMUM 
TEMPERATURE  IN  GASEOUS  EXPLOSIONS. 

Although  experimenters  are  unanimously  agreed  upon  the  fact  of 
the  suppression  of  heat  at  the  maximum  temperatures  produced  by 
gaseous  explosions,  they  differ  widely  in  their  explanation  of  the 
causes  producing  this  suppression. 

Three  principal  theories  have  been  proposed — 

1.  Theory  of  Limit  by  Cooling. — This  is  Hirn's  theory,  and  it 
assumes  that  when  explosion  occurs,  a  point  is  reached  when  the 
cooling  effect  of  the    enclosing  walls'  is    so  great  that  heat  is 
abstracted  more  rapidly  than  it  is  evolved  by  the  explosion,  and 
accordingly  the  temperature  ceases  to  increase  and  begins  to  fall. 

The  maximum  temperature  falls  short  of  what  it  would  do  if 
no  heat  were  lost  during  the  progress  of  the  explosion  to  the  walls. 
If  it  be  true  that  the  cold  surface  of  the  vessel  is  the  limiting 
cause,  then  the  maximum  pressure  produced  in  exploding  the 
same  gaseous  mixture,  in  vessels  of  different  capacity,  will  greatly 
vary.  When  the  vessel  is  small  and  the  surface  therefore  re- 
latively large,  more  heat  should  be  abstracted  and  lower  pressure 
should  be  produced.  This  is  not  the  case.  The  maximum  tem- 
perature produced  by  an  explosion  is  almost  independent  of  the 
capacity  of  the  vessel.  Surface  does  not  control  maximum  tem- 
perature, although  increased  surface  increases  the  rapidity  of  the 
fall  of  temperature  after  the  point  of  maximum  temperature. 

2.  Theory  of  Limit  by  Dissociation. — This  is  Bunsen's  theory, 
and  it  is  undoubtedly  largely  true.     The  fact  that  no  unlimited 
temperature  can  be  attained  by  combustion,  even  when  the  use  of 
non-conducting  materials  prevents  cooling  almost  completely,  is 
so  conclusively  established  by  science  and  practice  that  gradual 

s 


258  The  Gas  Engine 

combustion  due  to  dissociation  may  be  safely  taken  as  occurring 
to  a  considerable  extent  at  the  higher  temperatures  used  in  gas 
engines.  But  there  is  a  difficulty  in  its  application  to  all  cases. 
In  experiments  made  with  explosions  in  a  closed  vessel  the 
suppression  of  heat  is  almost  the  same  at  low  temperatures  as  at 
high  temperatures ;  thus  with  hydrogen  mixtures — 

Max.  temp,  of  explosion    900°  C.  ;  apparent  evolution  of  heat  55  per  cent. 
,,  ,,  ..  1700°  C.  „  ,,  ,,      54        ,, 

If  dissociation  were  the  sole  cause,  then  as  water  must  dissociate 
more  at  the  higher  temperature  than  at  the  lower,  the  apparent 
evolution  of  heat  should  be  less  at  1700°  C.  than  at  900°  C.  It 
is  not  so.  Some  other  cause  than  dissociation  must  therefore  be 
acting  to  check  the  increase  of  temperature  so  powerfully  at 
900°  C. 

3.  Theory  of  Limit  by  the  Increasing  Specific  Heat  of  the  Heated 
Gases. — Messrs.  Mallard  and  Le  Chatelier  have  advanced  the  theory 
that  up  to  temperatures  of  about  1800°  C.  dissociation  does  not 
act  at  all  or  only  to  a  trifling  extent.  They  consider  that  the 
gases  are  completely  combined  or  burned  at  the  maximum 
temperature  of  the  explosion.  But  the  specific  heat  of  nitrogen, 
oxygen,  and  the  products  of  combustion  increases  with  increasing 
temperature,  becoming  nearly  doubled  when  approaching  2000°  C. 
The  apparent  limit  is  due,  not  to  the  suppression  of  combustion 
as  required  by  the  dissociation  theory,  nor  to  the  loss  of  heat 
by  the  theory  of  cooling,  but  to  the  absorption  of  the  heat  which 
is  completely  evolved  by  the  increasing  capacity  for  heat  of  the 
ignited  gases.  The  same  objection  applies  to  this  as  to  the 
dissociation  theory.  If  it  were  entirely  true  that  specific  heat 
increased  with  increasing  temperature,  a  greater  proportion  of 
heat  would  apparently  be  evolved  at  the  lower  temperatures,  which 
is  not  always  the  case. 

It  is  impossible  to  discriminate  between  the  effect  produced 
by  increased  specific  heat  and  the  effect  produced  by  dissociation 
on  the  explosion  curves. 

Those  are  the  three  principal  theories  which  have  been  pro- 
posed, and  in  the  author's  opinion  none  of  them  completely  explains 
the  facts.  The  phenomena  of  explosion  are  very  complex,  and 


Tlieories  of  Action  of  Gases  in  Modern  Gas  Engine    259 

no  single  cause  explains  the  limit  and  other  phenomena  of 
gaseous  explosion.  These  phenomena  are  more  complex  than 
have  generally  been  supposed.  In  many  chemical  combina- 
tions it  has  been  proved  by  Messrs.  Vernon-Harcourt  and 
Esson,  and  Dr.  E.  J.  Mills  and  Dr.  Gladstone,  that  the  rate  at 
which  the  reaction  proceeds  depends  upon  the  proportions  ex- 
isting between  the  masses  of  the  acting  substances  present,  and 
those  neutral  to  the  reaction,  and  that  combination  proceeds 
more  slowly  as  dilution  increases.  From  this  it  follows,  that  in  a 
combination  where  no  diluent  is  present,  the  first  part  of  the 
action  is  more  rapid  than  the  last ;  at  first  all  the  molecules  in 
contact  are  active,  but  after  some  combination  has  occurred  the 
product  acts  as  a  diluent.  The  last  portion  of  the  reaction, 
having  to  proceed  in  the  presence  of  the  greatest  dilution,  is 
comparatively  slow.  Such  an  action  the  author  considers  occurs 
in  all  gaseous  explosions,  and  is  one  of  the  causes  preventing 
the  complete  evolution  of  all  the  heat  present  at  the  moment  of 
the  explosion. 

The  subject  is  a  difficult  one,  and  more  experiment  is  required 
for  its  complete  settlement. 


S2 


260  The  Gas  Engine 


CHAPTER  XL 

THE  FUTURE  OF  THE  GAS  ENGINE. 

SINCE  1860,  when  by  the  genius  and  perseverance  of  M.  Lenoir 
the  gas  engine  first  emerged  from  the  purely  experimental  stage, 
it  has  steadily  and  continually  increased  in  public  favour  and  use- 
fulness. At  first  more  wasteful  of  heat  than  the  steam  engine,  it 
is  now  more  economical ;  at  first  delicate  and  troublesome  in  the 
extreme,  it  is  now  firmly  established  as  a  convenient,  safe  and 
reliable  motor  ;  at  first  only  available  for  small  and  trifling  powers, 
now  really  large  and  powerful  motors  are  used  in  thousands. 
Many  inventors  have  contributed  to  its  progress,  but  its  present 
position  is  in  the  main  due  to  the  patience,  energy  and  command- 
ing ability  of  one  man — Mr.  Otto. 

In  1860,  the  efficiency  of  the  gas  engine  was  only  4  per  cent.;  in 
1886,  the  efficiency  of  the  best  compression  engines  is  18  per  cent. 

That  is,  at  first  a  gas  engine  could  only  convert  4  out  of  every 
100  heat  units  given  to  it  into  mechanical  work,  as  developed  in 
the  motor  cylinder  ;  now  it  can  give  18  out  of  every  100  units  as 
indicated  work. 

Having  advanced  in  economy  more  than  fourfold  in  the  past 
twenty-five  years,  what  limits  exist  to  check  its  progress  in  the 
future  ? 

Apart  from  the  greater  perfection  of  the  mechanical  arrange- 
ments of  the  gas  engines  of  to-day,  the  great  cause  of  improve- 
ment since  1860  is  the  successful  introduction  of  the  compression 
principle. 

Can  this  principle  be  much  further  extended  in  its  application? 
In  the  author's  opinion,  No. 

By  undue  increase  of  compression  the  negative  work  of  the 
engine  would  be  much  increased,  and  the  strains  would  become 


The  Future  of  the  Gas  Engine  261 

so  great  that  heavier  and  more  bulky  engines  would  be  required 
for  any  given  power.  Friction,  due  to  this,  increases  more  rapidly 
than  efficiency  ;  consequently  the  gain  in  indicated  efficiency 
would  be  more  than  compensated  by  loss  of  effective  power.  Im- 
provement must  be  sought  elsewhere. 

The  most  obviously  weak  point  of  the  present  engine  is  insuffi- 
cient expansion.  In  the  Otto  engine  the  exhaust  valve  opens 
while  the  gases  in  the  cylinder  are  still  at  a  pressure  of  30  pounds 
per  square  inch  above  atmosphere  ;  in  the  Clerk  engine  the 
pressure  is  sometimes  as  high  as  35  pounds  per  square  inch  above 
atmosphere  at  the  moment  of  exhaust. 

The  gas  engines  discharge  pressures,  without  utilising  them, 
with  which  many  steam  engines  commence. 

There  is  evident  waste  here,  which  can  be  remedied  by  using 
further  expansion.  In  continuing  expansion  the  loss  of  heat  to 
the  cylinder  would  not  be  so  great  as  in  the  earlier  part  of  the 
diagram,  because  the  temperature  is  greatly  reduced  ;  it  may 
therefore  be  supposed,  without  appreciable  error,  that  the  added 
portion  of  the  diagram  would  give  at  least  as  good  a  result  when 
compared  with  its  theoretical  efficiency  as  the  earlier  part.  If  the 
expansion  be  carried  so  far  that  the  pressure  falls  to  atmosphere, 
then  the  theoretical  efficiency  of  an  Otto  engine  would  be  0-5  ; 
theoretically  its  cycle  would  then  be  able  to  convert  50  per  cent, 
of  the  heat  given  to  it  into  indicated  work  ;  practically  the  com- 
pression gas  engine  at  present  converts  one-half  of  what  theory 
allows  ;  therefore  with  the  greater  expansion  it  may  be  expected 
to  give  one-half  of  50  per  cent. — that  is,  expansion  only  will  raise 
the  practical  efficiency  from  18  per  cent,  to  25  per  cent. 

By  complete  expansion  to  atmosphere,  the  gas  consumption 
of  an  Otto  or  Clerk  engine  could  be  reduced  from  20  cubic  feet 
per  I  HP  hour,  to  14/5  cubic  feet  per  IHP  hour.  There  are, 
of  course,  practical  difficulties  in  the  way  of  expanding,  but  they 
will  be  overcome  in  time.  Mr.  Otto  has  attempted  greater  ex- 
pansion in  various  ways,  and  so  has  the  author,  but  as  yet  neither 
has  succeeded  in  carrying  it  beyond  the  experimental  stage. 

It  must  not  be  supposed,  as  it  too  often  is,  that  a  high 
exhaust  pressure  means  an  uneconomical  engine,  or  that  compari- 
sons of  pressure  of  exhaust  give  the  smallest  clues  to  the  relative 


262  The  Gas  Engine 

economy  of  engines.  It  is  a  very  common,  but  a  very  erroneous, 
belief  that  if  the  pressure  in  the  cylinder  of  a  gas  engine  is  very 
near  atmospheric  pressure  when  the  exhaust  valves  open,  that 
fact  is  a  proof  that  the  engine  is  economical. 

This  is  not  so — indeed,  it  may  be  the  very  reverse. 

In  engines  of  type  3,  for  example,  in  which,  as  in  the  Otto  and 
Clerk  engines,  the  expansion  after  explosion  is  carried  to  the 
initial  volume  existing  before  explosion  and  no  further,  it  has 
already  been  shown  that  the  actual  indicated  efficiency  is  quite 
independent  of  the  increase  of  temperature  above  the  temperature 
of  compression.  That  is,  the  temperature  of  the  explosion  may 
be  anything  whatever  above  the  temperature  of  compression  with- 
out either  increasing  or  diminishing  the  indicated  economy. 

Suppose  an  Otto  diagram  with  three  expansion  lines,  (i)  max. 
temp.  600°  C.,  (2)  max.  temp.  1000°  C.,  and  (3)  max.  temp. 
1600°  C.,  the  maximum  temperatures  in  the  three  cases  being 
attained  at  the  beginning  of  the  stroke,  the  efficiency  of  these  three 
lines  is  identical.  Of  course  the  total  indicated  power  increases 
with  increase  of  temperature,  and  diminishes  with  diminution  of 
temperature,  but  the  proportions  of  the  heat  given  by  the  engine 
as  work  in  the  three  cases  remain  constant. 

The  same  thing  applies  to  any  number  of  intermediate  tem- 
peratures. 

It  might  be  supposed  that  the  line  i  by  expanding  more  nearly 
to  atmosphere  would  be  the  more  economical,  and  that  the  line  3, 
because  of  the  high  pressure  of  exhaust,  was  the  more  wasteful. 

It  is  a  peculiarity  of  this  cycle,  with  the  expansion  stated, 
that  the  efficiency  is  absolutely  dependent  upon  compression 
alone — that  is,  the  ratio  of  volume  before  and  after  expansion — and 
is  quite  independent  of  the  maximum  temperature. 

The  case  at  once  alters  if  expansion  be  carried  to  atmosphere. 
Here  the  line  3  would  give  far  greater  economy  than  the  others, 
and  efficiency  would  increase  with  increase  of  explosion  tempera- 
ture. 

Suppo-e  complete  expansion  successfully  applied  to  the  gas 
engine,  and  an  actual  indicated  efficiency  of  25  per  cent,  attained, 
can  any  further  improvement  be  hoped  for  ? 

What  causes  the  difference  still  existing  between  theory,  which 


The  Future  of  tJie  Gas  Engine  263 

shows  a  possible  50  per  cent.,  and  practice,  which  may  now  realise 
25  per  cent.  ? 

The  great  loss  is  heat  flowing  from  the  exploded  gases  through 
the  cylinder  walls.  Dr.  Slaby's  balance-sheet  of  the  Otto  engine 
shows — 

Per  cent. 
Work  indicated  in  cylinder  .....     i6'o 

Heat  lost  to  cylinder  walls 51*0 

Heat  carried  away  by  exhaust  .  .  .  .  31 'o 
Heat  lost  by  radiation,  etc.  .  .  .  .  2'o 

100 

By  expanding  as  described  it  would  be  altered  as  follows  : 

Per  cent. 
Work  indicated  in  c\  linder  .         .         .         .         .     25 -o 

Heat  lost  to  cylinder  wall- 51 'o 

Heat  carried  aw  ny  by  exhaust  .  .  .  .  22  x> 
Radiated  loss,  etc.  .  .  .  .  .  2  'o 

ICO 

The  work  done  will  be  increased  by  diminishing  the  loss  of 
heat  with  the  exhaust  gases,  but  the  loss  of  heat  to  the  cylinder 
walls  will  remain  constant.  This  assumes,  of  course,  that  the  in- 
creased time  of  expansion  is  balanced  in  loss  to  cylinder  walls  by 
more  rapid  rate  of  fall  ;  if  the  piston  velocity  is  not  increased  the 
result  will  not  be  quite  so  good.  If,  for  instance,  the  piston 
velocity  is  constant,  and  the  volume  to  surface  ratio  is  constant, 
the  expansion  will  only  give  results  as  follows  : 

Work  indicated  in  cylinder .  .  .  .  .21*0 
Heat  lost  to  cylinder  walls  and  radiated  .  .  66^5 
Heat  carried  away  by  exhaust  .  .  .  .  12*5 


lOO'O 


Expansion  so  arranged  as  to  be  equivalent  to  the  same  time  of 
present  piston  stroke,  o'2  seconds,  by  increasing  piston  velocity  and 
rearranging  cooling  surfaces,  will  give  25  per  cent,  of  total  heat  in 
indicated  work  :  if  surfaces  and  piston  speed  remain  unaltered,  so 
that  the  time  of  exposure  increases  in  same  ratio  as  expansion, 
then  21  per  cent,  only  will  be  attained.  With  proper  expansion, 
the  loss  of  heat  by  the  exhaust  gases  discharging  at  a  high  temper- 
ature may  be  greatly  diminished,  and  the  efficiency  would  be 
increased,  but  the  change  would  not  affect  the  loss  of  heat  to 
cylinder  walls  ;  it  would  even  increase  it 


264  The  Gas  Engine 

How  can  this,  the  greatest  loss  in  the  gas  engine,  be  reduced  ? 
The  loss  depends,  as  has  already  been  stated,  upon  the  ratio  of 
surface  to  volume  of  gases  exposed  to  cooling,  upon  the  time  of 
exposure,  and  upon  the  elevation  of  the  temperature  of  the  hot 
gas  above  the  enclosing  surfaces  cooling  it. 

It  is  evident  that  as  engines  increase  in  power,  the  capacity 
of  cylinders  of  similar  proportions  increase  as  the  cube  of  the 
diameter,  while  the  area  of  the  enclosing  cold  surfaces  increases  as 
the  square  of  the  diameter.  As  engines  of  greater  and  greater 
power  are  constructed,  the  surface  exposed  in  proportion  to  volume 
becomes  less  and  less ;  the  loss  of  heat  from  this  cause  will,  there- 
fore, diminish 

Increase  in  piston  velocity  will  also  diminish  loss,  by  diminishing 
time  of  contact :  300  feet  per  minute  is  the  usual  speed  at  present, 
and  it  cannot  be  advantageously  increased  in  small  engines,  as  the 
reciprocations  of  the  parts  become  too  frequent  for  durability  :  but 
in  large  engines  with  diminishing  reciprocation,  the  piston  speed 
may  be  increased  to  600  feet  per  minute,  and  still  be  within  the 
limits  practised  in  steam  engines. 

Increase  in  temperature  of  cylinder  walls  is  also  advantageous 
within  certain  limits.  The  author  has  found  a  difference  of  as 
much  as  10  per  cent,  upon  the  consumption  of  gas  of  an  Otto 
engine  when  at  17°  C.,  and  so  hot  that  the  water  in  the  jacket  was 
just  short  of  boiling  96°  C.  It  is  probable  that  still  higher 
temperature  could  be  advantageously  used,  but  there  is  a  limit 
imposed  both  by  theory  and  practice. 

However,  the  cycle  could  be  modified  to  permit  the  use  of 
very  hot  walls,  enclosing  the  gases  at  500°  C. 

When  all  these  precautions  against  loss  are  practised  in  large 
engines,  and  the  heat  loss  is  greatly  reduced,  another  complication 
steps  in,  which  modifies  the  theory  of  the  engine  very  considerably. 
That  complication  is  the  property  possessed  by  all  explosive 
gaseous  mixtures  of  suppressing  part  of  their  heat-  the  phe- 
nomenon of  Dissociation,  the  '  Nachbrennen  '  of  the  Germans,  or 
the  apparent  change  of  specific  heat  or  continued  combustion  of 
the  French  and  the  English. 

Although  a  gaseous  explosion  expanding  in  a  cold  cylinder 
behind  a  piston  doing  work  very  nearly  follows  the  adiabatic  line, 


The  Future  of  tJie  Gas  Engine  26$ 

yet  if  expanded  under  such  circumstances  that  the  loss  of  heat 
was  greatly  diminished,  it  would  no  longer  do  so. 

In  large  engines  the  expansion  curve  is  always  above  the  adia- 
batic  ;  in  small  engines  it  is  below  the  adiabatic. 

In  fig.  53,  diagram  taken  by  Professor  Thurston,  if  all  loss  of 
heat  to  the  cylinder  could  have  been  prevented,  the  expanding 
line  would  have  been  an  isothermal,  the  maximum  temperature  of 
1657°  would  have  been  sustained  to  the  end  of  the  stroke,  and  the 
actual  efficiency  of  the  diagram  would  have  been  0-40,  that  is, 
40  per  cent. 

At  the  point  7  the  temperature  would  be  1657°,  and  the  gases 
would  still  contain  60  per  cent,  of  all  the  heat  given  to  them,  and  if 
expanded  to  atmosphere  adiabatically,  the  combustion  being  sup- 
posed complete,  then  13  per  cent,  would  be  added,  making  a 
total  efficiency  of  53  per  cent 

If  the  loss  of  heat  through  the  cylinder  could  be  totally  sup- 
pressed, the  possible  efficiency,  taking  into  consideration  the 
properties  of  explosive  gases  is  53  per  cent.  It  is  impossible  to 
completely  avoid  loss  to  the  cylinder,  but  it  will  doubtless  be 
greatly  reduced. 

The  united  effect  of  expansion,  greater  piston  speed  and  reduc- 
tion of  loss  of  heat  to  the  cylinder  by  using  hot  liners,  when  carried 
out  in  an  engine  of  considerable  power,  would  cause  the  attainment 
of  a  practical  heat  efficiency  of  at  least  40  per  cent.,  and  this  with- 
out any  great  change  in  the  construction  of  gas  engines  now  made. 

Now,  how  do  these  efficiencies  compare  with  those  of  the  steam 
engine?  It  is  generally  admitted  that  the  best  steam  engines 
of  considerable  powers  and  of  the  latest  type,  when  in  ordinary 
work  do  not  give  an  efficiency  greater  than  10  per  cent.,  that  is, 
they  do  not  convert  more  than  10  per  cent,  of  the  heat  given  to 
'  the  boiler  in  the  form  of  fuel,  into  indicated  work.  In  small  engines 
of  such  powers  as  are  comparable  with  the  largest  gas  engines  yet 
constructed,  the  results  are  not  nearly  so  good,  an  efficiency  of 
4  per  cent,  being  a  good  result. 

The  reader  will  remember  that  the  term  efficiency,  as  used  in 
this  work  throughout,  is  defined  to  mean  the  proportion  of  heat 
converted  into  work,  to  total  heat  given  to  the  heat  engine. 

Efficiency  is  often  used  in  another  sense,  and  considerable 


266  The  Gas  Engine 

confusion  has  arisen  because  of  its  use  in  different  senses  by  diffe- 
rent writers.  In  comparing  engines  differing  in  their  nature,  the 
only  standard  of  comparison  possible  is  the  total  heat  or  total  fuel 
given  to  each  engine,  and  the  proportion  of  total  heat  or  total  fuel 
which  that  engine  can  convert  into  work.  The  source  of  power  is 
always  combustion,  and  the  temperature  of  combustion  may  always 
be  supposed  to  be  the  superior  limit  of  temperature  whatever  the 
working  process,  whether  steam  or  air  is  the  working  fluid.  From 
the  fact  of  taking  the  total  heat  as  the  basis  of  comparison,  the 
reader  is  not  to  infer  that  it  is  possible  even  in  theory  to  convert 
all  ot  it  into  work.  Professor  Osborne  Reynolds,  in  a  lecture  be- 
fore the  Institution  of  Civil  Engineers,  stated  that  this  seemed  to 
be  a  belief  popular  among  engineers  ;  the  author  does  not  think 
that  this  is  so. 

Certainly,  the  second  law  of  Thermodynamics  is  not  so  widely 
understood  among  engineers  as  it  should  be,  but  still,  few  suppose 
that  it  is  even  theoretically  possible  to  convert  all  the  heat  given 
to  an  engine  into  work. 

In  the  discussion  on  the  author's  paper  on  'The  Theory  of  the 
Gas  Engine,'  at  the  Institution  of  Civil  Engineers,  considerable 
confusion  arose  from  the  term  efficiency  being  used  in  different 
senses  by  different  speakers.  Professor  Fleeming  Jenkin  in  his 
lecture  very  clearly  defines  the  different  legitimate  uses  of  the 
term. 

Returning  to  the  comparison  of  gas  and  steam  engine  heat 
efficiency,  the  10  per  cent,  of  the  steam  engine  is  probably  very 
nearly  as  much  as  can  be  ever  attained  ;  it  may  be  exceeded  by 
using  high  pressures  and  great  expansion,  but  it  will  never  be  pos- 
sible to  attain  anything  like  20  per  cent.  The  limits  of  tempera- 
ture are  such  that  if  the  steam  cycle  were  perfect,  only  32  percent, 
of  the  whole  heat  could  be  converted  into  work ;  at  the  boiler 
pressures  and  condenser  temperatures  used,  the  theoretical  effi- 
ciency of  the  steam  engine  cycle  is  within  80  per  cent,  of  the  cycle 
of  a  perfect  engine,  that  is,  the  efficiency  theoretically  possible  is 
32  x  0-8  =  25-6  per  cent.  In  an  experiment  made  by  Messrs. 
B.  Donkin  &  Co.  on  a  63  HP  compound  engine,  the  results  as 
given  by  Professor  Cotterill  in  his  work  on  the  steam  engine  are 
as  follows  : 


The  Future  of  tlie  Gas  Engine  267 

Per  cent. 

Absolute  efficiency IIT 

Efficiency  of  a  perfect  engine       .        .         .         .     28*4 
Relative  efficiency 39 'i 

The  engine  received  100  heat  units  from  the  boiler  as  dry 
steam,  and  it  gave  IIT  units  as  indicated  work  in  the  cylinder. 
With  the  pressures  and  temperatures  given,  the  steam  engine  cycle, 
if  perfectly  carried  out,  falls  short  of  the  cycle  of  a  perfect  heat 
engine  between  the  limits,  so  that  22*7  per  cent  is  the  maximum 
efficiency  which  could  be  obtained,  supposing  no  other  loss  than 
that  due  to  the  imperfection  of  the  cycle.  The  cylinder  losses, 
condensation,  incomplete  expansion  and  misapplication  of  heat; 
make  the  actual  indicated  efficiency  IIT  per  cent,  so  that  half 
has  gone.  The  furnace  loss  diminishes  the  absolute  efficiency  to 
9 '2  per  cent,  and  it  is  extremely  improbable  that  improvement 
can  ever  increase  this  to  18  per  cent,  which  is  the  indicated 
efficiency  of  the  gas  engine  as  at  present 

It  is  impossible  that  the  steam  engine  can  ever  offer  an  effi- 
ciency of  40  per  cent.,  which  is  quite  possible  with  the  gas  engine. 

What  remains  to  be  done,  then,  in  order  to  make  the  gas 
engine  compete  with  steam  for  really  large  powers  ?  At  present 
the  largest  gas  engines  do  not  indicate  more  than  40  HP,  and 
very  few  are  in  use  so  powerful. 

The  gas  engine,  although  superior  in  efficiency  as  a  heat 
engine  to  the  steam  engine,  is  not  superior  in  economy  except  for 
small  powers,  where  steam  engines  are  very  wasteful  and  the  cost 
of  attendance  relatively  great. 

The  unit  of  heat  supplied  in  the  form  of  coal  gas  is  more 
costly  than  the  unit  of  heat  supplied  in  the  form  of  coal.  Gas 
producers  are  required  which  will  convert  the  whole  of  the  fuel 
into  gas  as  readily  as  steam  is  produced,  and  with  no  greater  loss 
of  heat  than  a  boiler  has. 

Mr.  J.  E.  Dowson's  producer  is  the  only  one  at  present  in 
existence  giving  suitable  gas,  and  it  requires  the  special  fuel 
anthracite. 

The  use  of  ordinary  fuel  has  not  yet  succeeded. 

A  good  gas  producer,  giving  gas  usable  and  free  from  tar,  is 
much  wanted. 


268  The  Gas  Engine 

But  when  all  this  is  done,  the  gas  engine  remains  in  some 
respects  inferior  to  the  steam  engine.  It  would  then  be  a  great 
advance  in  economy,  as  it  is  at  present  much  superior  as  a  heat 
engine,  or  machine  for  the  conversion  of  heat  into  work.  But 
mechanically  it  would  still  be  inferior  to  steam. 

As  a  piece  of  mechanism,  the  steam  engine  is  almost  perfect  : 
it  is  started,  stopped,  and  regulated  in  a  very  perfect  manner.  Its 
motion  is,  in  good  examples,  almost  perfectly  uniform  under 
variation  of  load,  and  but  little  fly-wheel  power  is  required,  be- 
cause there  is  little  or  no  negative  work. 

Its  motion  is  perfectly  under  control. 

The  gas  engine  itself  requires  much  improvement  in  this  re- 
spect ;  it  is  a  comparatively  inferior  machine  ;  at  best  it  receives 
only  one  impulse  every  revolution  when  at  full  power,  and  when 
under  light  loads  only  an  occasional  impulse. 

Means   must   be   found  to  make   it   double   acting,  and   to 
diminish  the  power  of  the  impulses  instead  of  diminishing  their  • 
frequency  for  governing. 

Means  must  also  be  found  to  start  and  stop  as  in  steam 
engines  ;  the  present  starting  gear  is  a  step  in  this  direction,  but 
requires  development. 

All  this  can  and  will  be  done;  it  is  a  matter  of  time  and 
patience.  It  can  and  will  be  made  as  mechanically  perfect  and 
controllable  as  the  steam  engine.  Flame  and  explosion,  seemingly 
so  untameable  and  destructive,  have  been  to  a  great  extent  tamed 
and  harnessed  in  present  engines.  Experience  is  growing,  by 
which  it  will  be  as  easily  and  certainly  directed  in  the  cylinder  of 
an  engine  as  steam  is  at  present.  The  furnace,  at  present  sepa- 
rated from  the  engine,  will  be  transferred  to  the  engine  itself,  and 
the  power  required  will  be  generated  as  required  for  each  stroke, 
and  the  system  of  storing  it  up  in  enormous  reservoirs — steam 
boilers — finally  abandoned. 

The  masses  of  smoke  polluting  our  atmosphere  will  be  entirely 
abolished  so  far  as  motive  power  is  concerned. 

The  author  cannot  do  better  in  conclusion  than  quote  the 
late  Professor  Fleeming  Jenkin,  expressing  his  belief  in  the  future 
of  this  form  of  motor. 

4  Since  that  is  the  case  now,  and  since  theory  shows  that  it  is 


The  Future  of  the  Gas  Engine  269 

possible  to  increase  the  efficiency  of  the  actual  gas  engine  two  or 
even  threefold,  then  the  conclusion  seems  irresistible,  that  gas 
engines  will  ultimately  supplant  the  steam  engine.  The  steam 
engine  has  been  improved  nearly  as  far  as  possible,  but  the 
internal-combustion  gas  engine  can  undoubtedly  be  greatly  im- 
proved, and  must  command  a  brilliant  future.  I  feel  it  a  very 
great  privilege  to  have  been  allowed  to  say  this  to  you,  and  I  say 
it  with  the  strongest  personal  conviction.' 


APPENDIX. 


ADIABATIC  AND  ISOTHERMAL  COMPRESSION  OF  DRY  AIR. 
(Professor  R.  H.  Thitrston,  Journal  of  Franklin  Institute,  1884.) 

One  hundred  volumes  of  dry  air  at  the  atmospheric  mean  temperature 
of  15-5°  C.  and  147  Ibs.  per  square  inch  undergo  change  of  volume 
without  loss  or  gain  of  heat.  The  temperatures  and  volumes  corre- 
sponding to  various  pressures  are  given.  Also  the  volumes  at  the 
various  pressures  if  the  temperature  remained  constant  at  15  -5°  C. 


Absolute  pressure  in 
Ibs.  per  sq.  inch 

Temperature  of 
compression  in 
Centigrade  degrees 

Volume  at 
temperature  and 
pressures  preceding 

Volume  if 
temperature  constant 
at  15-5° 

H7 

I5-5 

100  -0 

iS'o 

17-26 

98-58 

98-00 

20  -O 

42-60 

80-36 

73'50 

25-0 

6476 

68-59 

58-80 

30  'o 

82-10 

60*27 

49-00 

35  'o 

98-38 

54  "oi 

42-00 

40  "o 

113-86 

49-I3 

3675 

45  'o 

126-54 

45'i8 

3^-67 

50-0 

138-96 

4i  93 

29-40 

55  '° 

150  '53 

39'i9 

2673 

60  "o 

161  -38 

36-84 

24-5° 

65-0 

171-61 

34-80 

22-62 

70  'o 

181-29 

33  '02 

21  -OO 

75  "o 

190-49 

3i  '44 

19-60 

80-0 

199-26 

3°  '°3 

18-38 

85-0 

207-66 

2877 

17-29 

90-0 

214-71 

27-62 

I6-33 

95  'o 

223  '45 

26-58 

I5-47 

100  -o 

230-91 

25-63 

1470 

125-0 

264-66 

21-88 

11-76 

150  'o 

293-91 

19-22 

9'8o 

175  '0 

319-87 

17-23 

8-40 

2OO  'O 

343'3i 

I5'67 

7-35 

225-0 

36471 

14-41 

6-53 

250-0 

4II-57 

1338 

5'88 

300-0 

420-34 

"75 

4-90 

400-0 

480-76 

9o8 

3-90 

500-0 

531-21 

.     8-17 

2-94 

6OO  'O 

574  '93 

7-18 

245 

7OO'O 

603-74 

6-44 

2'IO 

800-0 

648-80 

5  '86 

1-84 

900-0 

680-86 

5  '39 

I-63 

IOOO 

710-49 

5-00      . 

1-47 

2OOO 

929-67 

3'o6 

0-74 

The  Gas  Engine 


ANALYSIS  OF  COAL  GAS. 
(T.  Chandler,   Watts'  'Diet.'  Supp.  3,  Part  i.) 


Heidelberg 

Bonn 

Chemnitz 

London 

Ordinary 

Cannel 

coal  gas 

gas 

vols. 

vols. 

vols. 

vols. 

vols. 

Hydrogen,  H      .     . 

44-00 

39-80 

51  '29 

46-00 

27-70 

Marsh  gas,  CH4  .     . 

38-40 

43'i2 

36'45 

39  '50 

50-00 

Carbonic  oxide,  CO  . 

573 

4-66 

4  '45 

7  '5° 

6-80 

Heavy  hydrocarbons 

7-27 

475 

4-91 

3-80 

13-00 

Nitrogen,  N    .     .     . 

4  '23 

4-65- 

1-41 

0*50 

0-40 

Carbonic  acid,  CO2  . 

o'37 

3'O2 

i  -08 

— 

O'lO 

Water  vapour,  H2O  . 

— 

— 

2  'CO 

2  "CO 

ANALYSIS  OF  LONDON  COAL  GAS. 
(Hvmpidge.) 


Sample  (A) 

Sample  (B) 

Hydrogen,  H  
Marsh  gas,  CH4  ..... 
Carbonic  oxide,  CO  ..... 
Olefines  

vols. 
50'05 
32-87 
12-89 
3-87 

vols. 
5  1  '24 

35-28 
7-40 
3-56 

2  '24 

Carbonic  acid,  CO2  ..... 

0-32 

0-38 

ANALYSIS  OF  BERLIN  AND  NEW  YORK  COAL  GAS. 


Berlin 

New  York  Municipal 
Gas  Light  Co. 

Hydrogen,  H 
Marsh  gas,  CH4 
f'arbonic  oxide,  CO 
Ethylene,  CoH4    . 
Nitrogen,  N 
Carbonic  acid,  COo 

vols. 

4975 
32-70 

o'54 
4'6i 
0-68 
2-50 

O'22 

vols. 
30'30 
24-30 
26-50 
15-00 
2-40 
I'OO 
OXO 

Appendix 


273 


ANALYSIS  OF  NATURAL  GAS  FROM  GAS  WELLS  IN  PENNSYLVANIA. 
(  Watts'  '  Diet,  of  Chemistry,'  Supp.  3,  Part  2.) 


Burns  Butler 
Co.  'swell 

Lechburgh 
Westmoreland  Co. 

Harvey 
Butler  Co. 

vols. 

vols. 

vols. 

Carbonic  acid,  COv> 

o'34 

°'3S 

0-66 

Carbonic  oxide,  CO 

trace 

O'26 



Hydrogen,  H 

6'io 

479 

I3'5° 

Marsh  gas,  CH4  . 

75  '44 

89-65 

80-11 

Ethylene,  C2H4    . 

18-12 

4  '39 

572 

Hydrocarbons     composition 

not  stated  .... 

— 

0-56 

— 

INDEX. 


ABE 

ABEL,  SIR  FREDERIC,  on  gun  cotton 

explosions,  88 

Actual  indicated  efficiency,  117 
Adiabatic  line,  40 
Air,  compression  lines  for,  40 
Air  engine,  Ericcson's,  24 

Joule's,  31 

Rankine  on,  24 

Stirling's,  25 

Wenham's,  25 

Air  and  gas  mixtures  : 

proportion  of,  99,  100,  101 

in  Clerk  engines,  193,  195 

in  Lenoir  engines,  128,  252 

in  Otto  engines,  173,  176 

in   Otto  and  Langen  engines, 

147 

Analysis  of  coal  gas — 

Berlin,  271 

Chemnitz,  271 

Deutz,  172 

Hoboken,  175 

London, 271 

Manchester,  109 

natural  gas,  272 

New  York,  271 

Apparent  indicated  efficiency,  117 
Atmospheric  engines — 

Barsanti  and  Matteucci,  u 

Brown's,  2 

Gillies',  151 

Otto  and  Langen,  136 

Wenham's,  35 

Atkinson's  differential  engine,  195 
Available  heat,  definition  of,  112 


BARNETT'S  compression  engines,  5,  6, 

9 

—  igniting  cock,  7,  207 
Barsanti  and  Matteucci  engine,  n 
Beau  de  Rochas  on  compression,  17 


CLE 

Berthelot  on  calculation  of  tempera- 
tures, 108 

—  explosion  pressures,  106 
wave,  114 

—  time  of  explosion,  114 
Berthelot  and  Vieille,  explosion  wave, 

87,  88 

Bischoff  engine,  132 

Bousfield  on  stratification,  250 

Boyle's  law,  38 

Brake,  tests  of : 

Brayton  engine,   157,  159 

Clerk  engine,  191-4 

Otto  engine,  172,  175,  180,  i8t 

Otto  and  Langen  engine,  141 

Brayton  engine,  20,  32,  152 

tests  of,  157,  159 

ignition,  217 

governor,  233 

petroleum  pump,  156 

Brooks  and  Steward's  trial  of  Otto  en- 
gine, 175 

Brown's  gas  vacuum  engine,  2 

Bunsen  corroborates  Davy's  experi- 
ments, 83 

—  on  explosion  pressure,  106 

velocity  of  flame  propagation,  84 

highest  temperature  of  combus- 
tion, 93 
dissociation,  257 


CALORIFIC  intensity,  90 

—  power,  90 

Cartridge  space  in  Million's  engine,  17 

Cayley's  proposed  engine,  25 

Charles'  law,  38 

Classification  of  gas  engines,  29 

Clerk  engine,  184 

tests  of,  191-4 

—  igniting  valves,  215,  217,  223 

—  governor,  233 


276 


TJte  Gas  Engine 


CLE 


FRE 


Clerk  starting  gear,  238 
Clerk's  explosion  experiments,  95 
Clutch,  Otto  and  Langen,  140 
Combustion  and  explosion,  79 

—  heat  evolved  by,  89 

—  volumes  of  products,  82 
Combining  weights,  80 
Compression  engines — 

Atkinson's,  197 
Barnett's,  5,  6,  9 
Brayton's,  20,  32,  152 
Clerk's,  184 
Million's,  16 
Otto's,  172 
Siemens',  18,  32 
Stockport,  197 
Tangye's  (Robson's),  195 
Compression,  Barnett  on,  5 

—  Beau  de  Rochas  on,  17 

—  Jenkin  on,  244 

—  Million  on,  16 

—  Schmidt  on,  17 

—  Siemens,  proposed  by,  17 

—  Witz  on,  244 

Critical  proportion  of  gas  in  mixtures, 

83 

Cushion  of  inert  gases,  247,  248 
Cycles  of  action,  29-35 


DAVY,  Sir  H. ,  on  inflammability,  82 
Deville,  St.  Claire,  on  dissociation,  92, 

93 

Deutz  coal  gas,  172 
Diagrams,  indicator,  Bischoff,  134 

Brayton,  158,  160 

Clerk,  192  5 

Hugon,  132 

Otto,  177,  179,  181 

Otto    and    Langen,    142,    147, 

148,  150 

Lenoir,  124,  125 

Simon,  164 

—  perfect  theoretical :  t 

—  type  i,  43 

—  —     2.  47 

3-  50,  52,  54 

i  A,  54 

Differential  engine,  195 
Dilution  of  mixtures,  83 
Dissociation,  Deville  on,  92,  93 

—  Bunsen's  theory  of,  257 

—  definition  of,  92 

—  Groves  on,  92 

—  Thurston  on,  178 
Drake's  engine,  10 

—  ignition,  10 

Dulong  and  Petit's  law,  90 


EFFICIENCY,  definition  of,  37 

—  of  perfect  heat  engine,  39 

—  of  imperfect  heat  engine,  41 

—  formulae,  56,  57 

—  apparent  indicated,  117 

—  actual  indicated,  117 

—  of  gas  in  explosive  mixtures,  112 
Efficiencies,  table  of,  68 

of  Brayton  engine,  162 

of  Clerk  engine,  192 

— •  —  of  Lenoir  engine,  128 

of  Otto  engine,  172,  176 

of  Otto  and  Langen  engine,  141 

Electrical  ignition,  203,  205 
Equivalent,  mechanical  of  heat,  36 
Enccson  engine,  fuel  used,  26 
Exhaust  gases,  temperature  of  Lenoir, 

122 

Otto,  173,  176 

Expansion  of  gases  by  heat,  38 
Explosion,  95,  115 

—  chemical  reactions  of,  81,  82 

—  Clerk's  apparatus,  95,  96 

—  combustion  and,  79 

—  observed  and  calculated  pressures, 
104,  1 06 

—  proportion  of  heat  evolved  by,  113 
Explosive  mixtures,  true,  79-82 

efficiencies  of  gas  in,  112 

curves  of  cooling,  97,  98 

inflammability  of,  83 

pressures  produced  by,  99-101 

flame  propagation  in,  81-89 

temperature  produced  by,  107- 

iii 
volumes  of  products,  82 

FLAME  propagation : 

Berthelot  and  Vieille  on,  87,  88 

Bunsen  on,  84 

Mallard  and   Le  Chatelier  on, 

85-87 

—  temperature  of,   93,   94,    108,  109, 

IIO,     CII 

—  theoretical,  temperature  of,  91 

—  temperature  of,  in  Lenoir  engine, 
126 

Brayton  engine,  161 

Otto  and  Langen  engine,  146- 

149 

Otto  engine,  177,  179 

Free  piston  engines — 

Barsanti  and  Matteucci,  n 

Gillies,  151 

Otto  and  Langen,  10,  136 

Wenham,  35 

type  i  A,  66 


Index 


277 


FRI 

Friction  of  Brayton  engine,  159 

—  Otto  engine,  174 
Furnace  engine,  Cayley,  25 
Wenham,  25 

—  loss  in  cylinder,  112 
Future  of  gas  engine,  260 

GARRETT'S  Governor  (Clerk  engine), 

234 
Gas,  coal  analysis  of — 

Berlin,  271 

Chemnitz,  271 

Deutz,  172 

Hoboken,  175 

London,  271 

Manchester,  109 

natural  gas,  272 

New  York,  271 
Gas  and  air  mixtures,  explosion  of, 

99,  100,  101 

—  best  proportions  of,  101-104 

Gas,    consumption    of    by     Bischoflf 

engine,  134 

Brayton  engine,  158 

Clerk  engine,  191-194 

Hugon  engine,  132 

Lenoir  engine,  124,  252 

Otto,  172,  175,  180,  183 

Otto  and  Langen,  141 

Gas,  efficiency  of  in   explosion  mix- 
tures, ii2,  113 
Gay-I.ussac's  laws,  82 
Gillies  engine,  151 
Governors  of  Bischoff  engine,  226 

—  Brayton  and  Lenoir,  233 

—  Clerk,  234 

—  Lenoir  and  Hugon,  226 

—  Otto,  230,  231,  232 

—  Otto  and  Langen,  227 

—  Tangye  (Pinkney),  235 

HAUTEFEUILLE'S,  Abbe",  engine,  i 
Heat,  available,  definition  of,  112 
table  of,  113 

—  balance  sheets  of  Otto  engine,  172, 
176 

—  engines,  perfect,  39 
imperfect,  41 

—  evolved  by  combustion,  88,  89 

—  mechanical,  equivalent  of,  36 

—  losses  in  gas  engine,  72 

—  lost  through  surfaces  in  Otto,  172 
176 

—  of  compression,  40,  270 

—  specific  of  gases,  90 

—  specific,  constant  volume,  89,  90 

—  specific,  constant  pressure,  89,  90 


MIL 


Heat, unit,  89 

Mini's  experiments  on  explosion,  104 
105 

—  theory  of  limit  by  cooling,  257 
Hoboken,  coal  gas,  175 

Hot  air  engines,  Ericcson's,  25 

Joule's,  25 

Rankine  on,  24 

Stirling's,  25 

Wenham's,  25 

Hugon's  engine,  20,  129 

—  igniting  valve,  209 
Huyghen's,  gunpowder  engine,  i 
Hydrogen,  80,  82 

—  mixiures,  84,  86,  87 

—  heat  evolved  by,  89 


IGNITING  arrangements,  202 

chemical,  225 

electrical,  203-207 

flame,  207-221 

incandescence,  222-224 

Imray  on  stratification,  250,  254 
Inert  gas,  cushion  of,  247,  248 
Inert  diluent,  246,  247 
Inflammability,  82 
Inflammation,  definition  of,  99 
Indicator   diagrams,    theoretical,    43, 

47,  50,  52.  54 

actual  (see  diagrams) 

Isothermal  line,  40 


JACKET,  water,  use  of,  27 
Joule,  Dr.,  hot  air  engine,  31 
Jenkin,  Prof.  Fleeming,  on  compres- 
sion, 244 
on  future  of  gas  engine,  268 


LANGEN'S,  Otto  and,  engine,  10,  136 

Lebon's  engine,  5 

Lenoir  engine,  13,  15,  30,  118 

—  electrical  ignition,  203 

Limits  of  dilution,  83,  100,  101,  226 

Limit  of  heat  evolution,  257-9 

London,  coal  gas,  271 

Losses  in  gas  engines,  72,  78 

Lubrication,  235-8 


MALLARD  and  Le  Chatelier's  experi- 
ments, 85-87 

theory  of  limit,  258 

Mechanical  efficiency,  Otto,  174 
Million  on  compression,  17 
—  gas  engine,  17 


278 


The  Gas  Engine 


MIX 


THE 


Mixtures,  true  explosive,  79-82 

—  best,  for  non-compression  engine, 
101 

—  dilute,  83 

Mixing  valve,  Clerk,  187 

Lenoir,  121 

Otto,  170 


NEUTRAL  gases,  cushion  of,  247,  248 
Non-compression  engines — 

Barsanti  and  Matteucci's,  n 

Bischoff  s,  132 

Gillies',  151 

Hugon's,  20,  129 

Lenoir's,  118 

Otto  and  Langen's,  136 

Street's,  i 

Wenham's,  35 

Wright's,  3 

Norton,  Prof. ,  on  Ericcson  engine,  26 
Notable  quantity,  246 


OILERS,  Otto's,  236 

Clerk's,  238 
Otto's  engine,  136 

governor,  228,  232 

igniting  valve,  242 

starting  gear,  242 

tests  of,  172,  175,  180,  181 

Otto's  theory,  245 

Otto  and  Langen's  clutch,  136 

engine,  136 

engine,  M.  Tresca  on,  145 


PACKED  charge,  247 

Papin's  experiments,  i 

Petroleum  engine,  Brayton's,  152 

Petroleum  pump,  156 

Pinkney's  governor,  156 

Piston  velocity — 
in  Lenoir  engine,  145 
in  Otto  engine,  172,  175 
in  Otto  and  Langen  engine,  144 

Pressures  and  temperature,  38,  107 

—  produced  by  explosion,  99-101 

if  no  loss  existed,  104-105 

Products  of  combustion,  82,  109 

proportion  : 

in  Hugon  engine,  131 

in  Lenoir  engine,  131 

in  Otto  engine,  173 


RANKINE  on  air  engine,  24 
available  heat.  112 


Rankine  on   science  of   thermodyn- 
amics, 36 

Ratio  of  air  to  gas,  in  explosive  mix- 
tures, 99-101 

in  Clerk's  engine,  193,  195 

in  Lenoir's  engine,  128 

in  Otto's  engine,  173,  176 

in  Otto  and  Langen's  engine, 

J4 
Ratio  of  compression  space : 

in  Clerk's  engine,  192 

in  Million's  engine,  17 

in  Otto's  engine,  172,  175 

Robson's  engine,  195 


SCHMIDT  on  compression,  16 
Schottler  on  stratification,  256 

—  tests  of  Otto  engine,  180 
Siemens'  proposed  compression,    18 

32 

Simon's  steam  gas  engine,  32,  163 
Slaby  on  Lenoir  engine,  248-9 

—  on  stratification,  247,  248 

—  tests  of  Otto  engine,  170-174,  180 
Slow  combustion,  Bousfield  on,  250 
Imray  on,  250 

Otto  on,  246 

Slaby  on,  247-9 

Specific  heats  of  gases,  90 
Starting  gear,  Clerk,  239 

—  Otto,  242 
Stockport  engine,  197 
Stratification,  Bousfield  on,  250 

—  experiments  in  support  of,  249-250 

—  fallacy  of,  254,  255,  256 

—  Lenoir  on,  16 

—  Otto  on,  246 

—  Schottler  on,  256 

—  Slaby  on,  247,  248 
Street's  gas  engine  pump,  i 


TABLE  of  efficiencies,  68 
Tangye  engine  (Robson's),  195 
Temperature  of  combustion  in  Bray- 
ton  engine,  161 

—  exhaust  in  Lenoir's  engine,  122 

Otto's  engine,  173,  176 

Temperatures  of  explosion,  107-111 
in  Lenoir's  engine,  125,  126 

Otto  and  I.angen  engine,  146- 

149 

—  —  Otto  engine,  177,  179 
Theoretic  efficiencies,  68 
Theories  of  actions  in  cylinder,  243 
Thermodynamics  of  the  gas  engine, 

36 


Index 


279 


THU 


Thurston  s  experiments  on  Otto   en- 
gine, 175 

—  on  dissociation,  178 

Tresca's   experiments  on  Lenoir  en- 
gine, 123 

Hugon  engine,  132 

Otto  and  Langen  engine,  141 

—  theory  of  Otto  and  Langen  engine, 

145 

Type,  first  description  of  perfect  cycle, 
29 

—  second  description  of  perfect  cycle, 

3° 

—  third  description  of  perfect  cycle, 

32 

—  i  A,  description  of  perfect  cycle,  34 

—  i  : 

Lenoir  engine,  118 
Hugon  engine,  129 
Bischoff  engine,  132 

Bray  ton  petroleum  engine,  152 
Simon  engine,  163 

—  3:  165 

Otto  engine,  166 
Clerk  engine,  184 


WRI 


Type  3  (continued) 

Tangye  engine,  197 

Stockport  engine,  197 

Atkinson  engine,  199 
—  i  A: 

Otto  and  Langen  engine,  136 

Gillies  engine,  151 


VACUUM  gas  engine,  2 
Velocity  of  flame  propagation,  85 
Volumes  and  relative  weights  of  gases, 
81 

of  Deutz  coal  gas,  172 

of  Hoboken  coal  gas,  176 


WATER  jacket,  use  of,  27 
Wave,  explosion,  114 
Wedding  on  dissociation,  183 
Weights    and  volumes,    relative,    of 

gases,  81 

Weights,  molecular,  of  gases,  81,  82 
Wenham's  engines,  25,  35 
Witz  on  compression,  244 
Wright's  engine,  3 


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